Planar Campaigns

The typical adventurer can readily secure fame and fortune within a single dungeon, nation, or continent, and for many campaigns, sticking to an easily grasped fantasy world of goblins, knights, and near-Earth conditions is the right move. After all, traveling the multiverse increases the possibilities and dangers by untold orders of magnitude.

In capable hands, a planar campaign can inspire stories unlike any other. But if handled poorly, a romp across the planes can turn into a disjointed series of accidents or sudden endings that could potentially harm a long-term game. So how can you as a GM convey a creative, fun, and memorable experience?

Why The Planes?

Okay, you’ve made the choice to take your game to the planes, but your campaign’s characters already enjoy stabbing bad guys on their home world. So why should your players risk life, limb, and the integrity of their souls to travel the planes when there are perfectly good dungeons in their own corner of the cosmic backyard? The possibilities are limitless, yet most goals fall within four categories: access, acquisition, confrontation, or rescue.

Each of these can be the ultimate aim for a campaign or merely a step toward a bigger task, and an effective planar campaign uses several of these goals to create a compelling sequence of events.

Access Just as adventurers might negotiate to use a rare library, discover a sage’s secrets, or learn a forbidden technique on the Material Plane, so too might they bargain as they seek similar lore, tools, and resources on another plane. Outsiders found commonly on the planes are often ageless immortals, boasting divine connections, forms, and faculties that are far beyond those of mortal humanoids. With their superior capacity and abundant time, these beings create unparalleled workshops for crafting phenomenal artifacts, ironclad vaults that contain unspeakable mistakes, and halls for conveying the perfect nuance of every performance type.

From the PCs’ perspective, these are tools that can enable them to achieve otherwise impossible goals. A lost secret might survive only in an extraplanar library. The conditions of an evil artifact’s destruction might require travel to the heart of a divine realm. Learning an antagonist’s plans might require partaking in an extraplanar intrigue in which the PCs must survive alien politics and interview long-dead souls. Sometimes the goal is to journey to a Transitive Planes with the intention of accessing an obscure demiplane or other realm that conventional magic can’t find.

Acquisition Even the most geologically varied planet pales in comparison to the unparalleled diversity possible on just one of the nigh-infinite planes. Most other planes consist of the distilled essence of an alignment or element, each of which provides an extraordinary substrate to nourish a dazzling array of organisms and propagate realities. Moreover, many planes host realms that are occupied continually by inventive creatures who spend untold eons creating countless tools, structures, and masterpieces that surpass anything a Material Plane world might produce. As the realms of the gods themselves, the planes can be home to utterly unique treasures that represent the very first or purest form of a given object.

Of course, adventurers specialize in studying and acquiring baubles, so to some, a plane’s treasures may simply be awaiting harvest by the right hands. The fabled artifact necessary to slay a demon lord might reside in Heaven— perhaps in the possession of an archon who’s not inclined to share. The reagents needed to brew a powerful potion to end a mythic curse might grow only in Abaddon’s tear-soaked soil or crystallize in the Plane of Earth’s most secluded caverns. Some travelers might seek real estate instead of treasures, skirmishing with the Astral Plane’s shulsagas to claim a newly formed demiplane as their own.

Confrontation

Outsiders are among the oldest and most dangerous creatures in the multiverse, and many interfere with mortal worlds to secure devotion for their divine patrons, favorably aligned souls, more power, or any combination of the three.

From their extraplanar bastions, predatory outsiders can plague other realms with near impunity, working through lesser beings and cults dedicated to enacting their patrons’ wills. If adventurers call such a being their nemesis, the only solution may be to track down the creature’s home and destroy it there.

That’s easier said than done, however. These beings often have a host of allies defending a well-fortified abode. Heroes leading the charge are unlikely to share their enemy’s alignment or subtypes, potentially exposing the attackers to hostile planar traits. The strongest outsiders can even wield the power of their home planes as a weapon, much as demon lords wield mythic power within their own realms. If adventurers can destroy such beings, they can end countless plots at once and create a power vacuum felt across the planes.

Rescue

Two facts bear repeating: the planes are dangerous, and souls are valuable. Unpredictable and titanic weather events can shatter even the mightiest magical vehicles or scatter travelers. Plane-hopping magic can fail and send someone to a random destination, while the death of a spellcasting ally might mean the loss of a group’s only ticket home.

Even something so mundane as a cave-in can leave otherwise able explorers stranded on another world. Rescuing allies or loved ones from such a fate may be the only way to save their lives—or failing that, might become the only way to allow their souls to reach the intended afterlife. Some of the most abrupt disappearances are those dealt by the Donjon card found in a deck of many things.

Any number of fiends and villains are all too happy to capture living prey to consume, experiment on, sacrifice, or torture. Among the most infamous abductors are the Ethereal Plane’s xills and the far-ranging devourers that prey upon live hosts and mortals’ souls, respectively.

Night hags are practiced soul trappers who might trade a hero’s soul away or consume it, preventing resurrection unless intercepted. Devils’ use of infernal contracts can let them abscond with a PCs’ friend or ally under the wrong circumstances (or insidiously small print).

Presenting The Planes

There’s a significant reason that most campaigns take place on an earth-like world: it’s predictable. Gravity works as expected, there are 24 hours in a day, seasons exist, and weather patterns make sense. That changes rapidly the moment the PCs start jumping to other planes of existence, and conflict between the players’ expectations and each planar reality is often more jarring for you than for them, due to potential challenges managing and presenting those differences in compelling ways. The ways in which you can overcome these difficulties vary as much as the planes themselves, but the basics fall into three broad concepts.

Not Just Another Dungeon

Wherever the PCs go, there will be creatures, dangerous sites, and places to congregate—the dungeons, NPCs, and settlements that are central to nearly every adventure. The temptation might be to just reskin these features to match the plane’s flavor, but that superficiality is insufficient; the plane should be an integral part of the setting and the players’ experiences. That includes everything from the textures of surfaces to the planar traits that rewrite reality to the ways in which the plane’s cultures (or other inhabitants) adapt to meet these odd standards.

This is especially true in the Outer Planes, where powerful alignment traits can shape environments and many inhabitants have gods for neighbors.

As an example, filling a dungeon that is located in Hell with devils is a disservice. Instead, consider what abilities are commonly available to local creatures (e.g., teleportation) and how those abilities might shape the denizens’ behaviors in this infernal citadel. If the site is constructed from evocative materials, with walls made from ever-burning bones or floors composed of tombstones, don’t stop at asking why it’s that way—think about what effect those choices have on the intended inhabitants and the intruding PCs. If there’s a fiend-operated market nearby, consider acceptable currency, how the traditional marketplace interactions might differ from those of the PCs’ home world, what’s being sold, and who the expected customers are (outsiders have fairly limited needs, after all).

Normal Is Abnormal

The planes often subvert common cultural and physical expectations. Depending on the place, the PCs might have to navigate cities designed entirely for locals who can fly, swim, or spend extensive periods of time in gaseous form.

Violent thoughts might physically manifest and harass the thinker, or raw chaos might reshape objects left unattended for too long. To locals, these occurrences might be part of their everyday lives, and this nonchalance can be as alien as the phenomena.

Traveling the planes exposes the PCs to jaw-dropping creatures and traditions, and there’s no reason their presence wouldn’t have a similar effect on the destination’s citizens.

Depending on the location, native creatures might find the PCs’ appearances, behaviors, and bodies to be unsettling or appalling. Such inhabitants might demonstrate intentions entirely through pheromones and color displays, have superstitions about blinking eyes, see the world in a wider range of colors, or be unable to experience certain emotions.

By contrast, cosmologically cosmopolitan regions might be familiar with travelers from the PCs’ home world, and some planar metropolises boast foreign districts where offworld creatures are more welcome. An introductory encounter set in such a space has the benefit of allowing the PCs to start in a less jarring place and work their way into more bizarre regions at their own pace.

Making the abnormal normal comes with an important warning: be very careful about rewriting the laws of reality. It might seem like a grand idea to assume that physics, down to the atomic level, operates differently on another plane, but that would mean even a 20th-level wizard’s body could discorporate in an instant. For most groups, aim for a middle ground of unsettlingly odd— an exciting destination that doesn’t entirely upend the game’s mechanics. Like running an intense investigation or a horror campaign, striking the right balance for a planar adventure might involve a conversation with your players to understand everyone’s boundaries and expectations.

Traveling The Planes

A manageable planar adventure thrives on direction— especially as the PCs and players start to travel the planes for the first time. The more familiar the players become with all forms of planar exploration and the wider their circle of planar contacts becomes, the more flexibility you can provide.

Second to a compelling story, the best tool you have for steering the action is the difficulty of traveling from one plane to another. Most adventuring groups lack the means to move between planes independently until around 9th to 13th level, so they’re otherwise reliant on extant portals that the you introduce and control, favors from outsiders, and rare magic items. PCs in a planar campaign might receive one or more such tools as treasure above and beyond their expected wealth by level, with the understanding that these are plot-critical rewards that enable future adventures. The following is an overview of well-known tools that the PCs might acquire.

Amulet of the Planes: No item is as unambiguously effective for planar travel as this amulet, which allows an entire group to journey together to a wide range of destinations.

Cubic Gate: With only six possible destinations, a cubic gate is an ideal tool for planar campaigns of limited scope.

Flying Skiff: Few options are as luxurious and expensive as a flying skiff; entire campaigns might be built around this device. The PCs might serve as crew aboard a flying skiff and travel to sundry destinations under their employers’ commands, only to find themselves inheriting the ship when misfortune befalls the previous owner.

Obsidian Steed: This figurine of wondrous power is an excellent escape plan—as long as players account for the fact that it can carry only a single creature. The item’s tendency to deposit good-aligned riders in unpleasant places could also be the basis for a time-critical rescue mission.

Planar Keystones: These items make for a low-cost method to allow travel to and from specific planes. Since their destinations are limited and it takes time to activate them, planar keystones have no real use in combat. But as a method of transporting a group of PCs to or from a plane at any experience level, planar keystones are one of your best options.

Robe of Stars: A robe of stars allows only one creature to travel. However, because the robe physically transports the wearer to the Astral Plane, they’re neither protected by nor limited by the silver cord afforded by the astral projection spell, allowing limitless travel from there to any number of demiplanes and Outer Planes.

Staff of the Planes: When fully charged, this staff can transport a group to another plane and back or transport the group while protecting up to five of its members from dangerous planar traits. In an emergency, the staff is also a way to escape a dangerous or escalating situation. Keep in mind that only a fairly high-level spellcaster can recharge the staff, though for some campaigns, a limited resource like this is exactly what’s needed.

Well of Many Worlds: Particularly useful for sandbox-style campaigns, this portable gateway allows nearly limitless travel. However, it’s important to keep in mind that a well of many worlds can be closed or moved only from the side of its origin, not from the realm into which it opens. That means that if the PCs rely on the well for their return trip, they will be forced to leave it open and hope that no dangerous denizens slip through the gate (or worse, close the well and take it away) before they can return. Alternately, the first stage of a quest might require the PCs to navigate a hostile region to reach a well’s launching point.

Desperate Measures: Several other items facilitate planar travel but in extraordinarily dangerous and unpredictable ways. Even so, circumstances might leave the PCs no choice but to place a bag of holding inside a portable hole or destroy a staff of power in the hopes of going somewhere— anywhere—else.

Planar Tuning Forks

Plane shift requires a specific tuning fork attuned to the destination plane, and while the Core Rulebook lists this as a spell focus with no listed price (implying that each spell component pouch has a full array of tuning forks), that should not be the case for any campaign that features planar travel. Limiting the PCs’ access to specific planes’ focuses can help manage where the PCs can travel to. Finding the right fork—or uncovering an unidentified fork and exploring whatever exists at its attuned destination—could be the basis for an adventure in its own right.

You should strongly consider assigning gold piece values to the tuning forks required for using plane shift to travel the planes. Not only does this give you greater control over which planes the characters can access, but it also creates an interesting and even collectable commodity for planar travelers. By assigning the following values, you also prevent tuning forks from being automatically available with the purchase of a spell component pouch, with a side benefit of easily indicating which might be available for purchase in settlements.

Note that some planes, like the Akashic Record, cannot be traveled to via plane shift at all, while others, like the Dimension of Time, are notoriously difficult to create lasting tuning forks for.

Table 2–2: Plane Shift Tuning Forks

Item	Price
Untuned fork	25 gp
Common tuning fork	100 gp
Uncommon tuning fork	2,000 gp
Rare tuning fork	20,000 gp
Unique tuning fork	Priceless

Planar tuning forks have no notable weight.

Untuned Fork

An untuned tuning fork appears as a plain iron tuning fork and cannot be used on its own as a focus for plane shift. An untuned tuning fork can be crafted as a mundane item with a successful DC 20 Craft (jewelry) check, but only if the artisan also has at least 1 rank in Knowledge (planes).

Before an untuned fork can be used as a focus for casting plane shift, it must be physically transported to the plane in question and subjected to a specific condition while on that plane, as detailed for each category of tuning fork. Once a previously untuned fork is attuned to a plane, it changes appearance to reflect that plane’s nature and cannot be attuned to a different one. Regardless of the plane in question, only one untuned fork can be attuned at a time in a 300-foot radius. Untuned forks take time to attune to a plane, and if a second untuned fork is struck within 300 feet of another fork that is in the process of becoming attuned, planar dissonance causes the attuning process for both forks to immediately end, and the procedure must start anew.

Common Tuning Fork

A common tuning fork is one attuned to any of the 19 planes. For layered planes, a common tuning fork can be attuned only to the first layer of the plane. Attuning a common tuning fork requires striking it against a solid surface somewhere on the plane, after which the tuning fork must be allowed to absorb the plane’s energies for 24 hours. Once this time passes, the untuned fork becomes attuned to that plane.

Uncommon Tuning Fork

An uncommon tuning fork is one attuned to a major demiplane or to a further layer of an Outer Planes. Planes mentioned in this book that require uncommon tuning forks include any of the deeper layers of Heaven, Hell, or the Abyss; stable dreamscapes in the Dimension of Dreams, such as Leng (plane shift does not allow travel to most dreamscapes).

Attuning an uncommon tuning fork is done in much the same way as a common tuning fork, but the tuning fork must be struck against a magic item of CL 11th or higher that was created on that plane, a creature of CR 11 or higher native to the plane, or a unique or specific feature of the plane as determined by the GM—such features are generally protected and should require mid-level play (9th level or above) to access. Once so struck, the tuning fork must be allowed to absorb the plane’s energies for 3 days.

After this time passes, the untuned fork becomes attuned to that plane.

Rare Tuning Fork

A rare tuning fork is one attuned to an obscure demiplane, a demiplane that is simply difficult to travel to due to its nature, or a demiplane that is particularly small and insignificant.

Planes mentioned in this book that require rare tuning forks include the DeadLands, the Harrowed Realm, and The Apocalypse Archive.

Attuning a rare tuning fork is done using the same method as an uncommon fork, but the fork must be struck against an artifact created on that plane, a creature of CR 21 or higher native to the plane, or a unique or specific feature of the plane that you determine—such features are generally well guarded and difficult to reach, and thus they should require relatively high-level play (15th level or above) to use. Once so struck, the fork must be allowed to absorb the plane’s energies for 1 week. After this time has passed, the untuned fork is attuned to that plane.

Unique Tuning Fork

For lost or forgotten demiplanes or deliberately difficult-to-reach dimensions, the appropriate tuning forks should be priceless. Their acquisition forces the players either to go on a quest to locate such a tuning fork or to bring an untuned fork to the plane in question, traveling there via other means, to attune the fork for later use. Once the party acquires such a tuning fork, you should use the prices provided on Table 2–2: Plane Shift Tuning Forks as a guide to determine the tuning fork’s value, keeping in mind that if the PCs flood a market, the fork’s value will decrease. Moreover, powerful denizens of such realms might take offense to the availability of keys to their empires and may attempt to deal with the spread of attuned forks. As a general rule, if the plot of an adventure (be it a published one or an adventure of your own design) hinges on the PCs’ being trailblazers to a forgotten or obscure demiplane, the demiplane should be counted as a unique one.

Attuning an untuned fork to a unique plane or dimension requires a specific task for that plane. For example, attuning a fork to a legendary vampire’s personal demiplane might require a player to plunge the tuning fork into the heart of a vampire native to that plane and then allow the fork 13 days to absorb the planar energies before it becomes fully attuned.

Alternate Cosmologies

This game assumes a cosmology centered on the Great Beyond, a concept that not only contextualizes the alignment system, the functionality of certain spells (such as etherealness and shadow walk), and the nature of the divine, but also helps to define the roles of every outsider in the game. However, the Great Beyond is far from the only possible model for a cosmology in your game. Presented below are several alternate cosmologies you might wish to explore for your own setting. Consider these examples as options and inspirations; as always, you are encouraged to alter the game world to fit your desires and expectations, as well as those held by your players.

Alignment-Free Cosmology

The Great Beyond relies on the presumption of outsiders linked to alignment, but what happens if you remove alignments from the game completely? In this alternative cosmology, the various outsider races might represent not alignments but concepts. Archons represent justice, while angels serve as unique representatives of their divine patrons, agathions as mercy, azatas as freedom, inevitables as rules, proteans as change, devils as punishment, daemons as death, and demons as temptation to sin. You can delve even further, assigning (for example) each individual type of devil a different form of punishment. From such decisions, entire new planes suggest themselves—one for each race of outsider to call home. Regardless of the conceptual assignments used, it’s not likely that large amounts of flavor change heavily, but the mechanical framework requires adjustments in the absence of spells and abilities that rely solely upon alignment, such as those offering protection from evil or good or from chaos or law.

In an alignment-free cosmology, rivalries between outsiders would not be split upon the easy lines of alignment, and unusual alliances might be made. Daemons representing death by execution might collaborate with devils representing lawful punishment and archons devoted to justice. Even stranger alliances might form between outsiders associated with more narrow concepts, such as a type of protean devoted to creative architecture working alongside an axiomite devoted to city planning.

Without alignment to rely on, the gods in this cosmology play a more important role, rallying a multitude of outsiders to the concepts encompassed by their areas of concern and enabling campaigning based on each type of outsider’s role rather than its morality and ethics. Mortal souls still flow to the specific god the owner worshiped above others, but rather than alignment, the owner’s actions serve as the determining factor of her fate if she died without a specific faith. An honest farmer’s soul would migrate toward the realm of the inevitables, perhaps, since the farmer obeyed the laws of the land and dutifully provided for her family and society, whereas another farmer might go to the realm of devils because of his penchant in life for abusing his employees.

In such a world, spells that function based on alignment will work quite differently, or perhaps not at all. In most cases, this puts you in the position of having to make decisions on the spot as to whether detect evil would provide information, or whether or not a holy sword might hurt the creature that attempts to wield it. As a result, the players’ trust in you becomes far more important in an alignment-free cosmology, since they must respect and trust that you are making decisions for the good of gameplay, not to be merely antagonistic or competitive.

Alternate Realities and Parallel Worlds

The universe is a brilliant and multifaceted jewel, with each angle and cut offering a different reflection of existence. Many of the countless faces may look similar, but each is fundamentally different, containing slightly or significantly changed versions of reality. Each of these alternate realities fills a role vacated by the loss of other planes, follows different metaphysical rules, and has natives comprised of one or more specific outsider races.

Alignment serves as a linking factor for outsider species but lacks the implications of a broader cosmic force with effects for mortal souls.

Archons might rule one version of reality and travel to others seeking to spread their creed of organized benevolence, guiding some worlds’ development, defending others from rival forces, and perhaps even conquering those impossible to redeem by subtler methods. Daemons could hail from a reality wherein they have already accomplished their goal of obliterating all mortal life. Now, from a cosmos lurching towards entropic heat death—the stars long ago consumed by ravenous daemon gluttons akin to malevolent black holes—daemons seek to spread outward to obliterate each life-form extant in adjacent facets of reality. Proteans, as beings of chaos, would swim freely between facets of the world, belonging to all and none at once, and defy the rules of the cosmos while remaining true to their nature.

Gods in this cosmology exist in their own alternate realities. Perhaps they are the living result of those worlds’ souls merging into singular beings, or, in a world where belief had the power to alter the fundamental structure of reality, gods could be born out of gestalt mortal belief. In other facets, outsiders and gods might hold no sway at all.

Each instance of reality is, in effect, an opportunity for you to create entirely new settings for the players to explore.

One intriguing possibility with this cosmology is that there could exist multiple incarnations of each character across multiple worlds. Thus, when a PC dies and is resurrected, the restored body is not so much brought back to life as it is pulled from another reality, so each time a character returns from death, she might change in strange and unexpected ways. This gives a player the opportunity to completely rebuild her character upon resurrection, explaining, for example, that the new incarnation was drawn from a world where the character trained not as a wizard but as a fighter.

Dualistic Cosmology

Not every belief system contains a multitude of gods. Some involve only a pair of diametrically opposed divine rivals, both competing for worshipers’ souls to either reward or punish. In this model, a dualistic cosmology would consist of only two Outer Planes, one for each divine force. The precise nature of those planes would rely entirely upon the nature of the gods in question.

In one example, there is only a Heaven and Hell, with the mortals who lived a good life ascending to Heaven among the archons and the wicked descending to Hell to receive punishment at the hands of devils. The role of law and chaos is less relevant than that of good and evil in this setting, and any pair of the good and evil planes could stand in for Heaven and Hell. With one deity ruling each plane, the role of demigod becomes one of servitude, each demigod being an extension of the deity’s will. A third, intriguing category could exist in the form of demigods who adhere to neither side of this eternal conflict, introducing such a category could erode the draw of a dualistic cosmology.

Rather than good and evil, a dualistic system based on law and chaos might have a single plane of law and a single plane of chaos complete with a variety of lawful and chaotic outsiders. Depending on their actions and beliefs while alive, adherents of a deity of law might ascend to a lawful plane at the moment of their death, only to be punished for transgressions. By contrast, adherents of chaos might find themselves hurled into a realm that combines the whimsies of proteans alongside the destructive horrors of demons and the freedom-loving passions of the azatas.

This dichotomy need not adhere to the concept of alignment at all; instead, its deities could embody other dualistic concepts. For example, a deity of day could vie against a deity of night, with both having positive and negative areas of concern in their portfolios. This system might have one divine power enthroned within the Fey World and its rival dwelling in the Shadow Plane. Life and death. Love and hate. Fire and water. The possibilities are boundless.

Extraterrestrial Planes

Rather than arriving through portals from other planes, outsiders hail from the depths of space, their homes farflung planets. These outsiders are diverse, strange alien races that are genuinely physical beings that originate from bizarre worlds. Archons manifest as a race of beings devoted to implementing their own vision of benevolent order and structure upon other similarly mortal worlds, while devils oppose their archon creators as a splinter group devoted to conquest—or they could be an entirely different race of aliens from another solar system that has long fought the archons. Proteans could be serpentine beasts that swim the depths of space, delighting in the chaos and anarchy of a cosmos that predated the formation of planets and other races. Demons might be a race of bloodthirsty, starfaring marauders that come from worlds that have been destroyed, while inevitables could be a robotic species devoted to ensuring the lawful progression of the cosmos as given to them by their long-absent original creators.

Souls in such a cosmology may hail not from the Positive Energy Plane but from a vast and distant cosmic phenomenon linked to the birth of the cosmos that still sheds souls into creation. Similar to the cycle of mortal souls in the Great Beyond, souls migrate from this point and filter out to a myriad of worlds around a multitude of stars. The role of the Negative Energy Plane might then be filled by vast black holes that churn at the fringe of the universe, with ghosts, ghouls, and other undead drawing their power from these immense, haunted stars.

Upon death, rather than migrating to the realms of gods or alignment-based planes to receive their final reward or punishment, souls would instead travel to the planet or solar system claimed by their god or roam elsewhere to join those beings who were most metaphysically associated with their actions and beliefs in life. A chaotic neutral woman with no religious beliefs might die only to reopen her eyes and find herself swimming through deep space, reborn as a protean amid a chorus of her kind. By contrast, a penitent man with deep belief in his goddess might die and awaken as a petitioner at her feet, staring up at a sky with alien stars—only to dwell physically there with her for eternity.

In this cosmology, the gods might simply be ultrapowerful aliens who rule planets or entire solar systems reserved for the souls of their followers, who remain protected from the dangers and terrors between the stars for the rest of their eternal existence. Faith in these entities that dwell in a physical location rather than an unreachable plane might prove less fervent, or their proximity could make devotion stronger, even allowing for commonplace divine contact— unless the gods united in forbidding it.

Mundane Cosmology

What if there were no other planes at all, in any form whatsoever? What if the universe was all there was, with nothing beyond the prosaic existence of physical reality? No celestials, no fiends, no elementals, and no gods? In such a setting, life would be followed by death and the oblivion of nonexistence. What sort of world would exist in the absence of the celestial, infernal, and even the divine? Without the ever-present threat of divine justice and an eternal reward or punishment, what would such a world look like? Religion would most certainly still exist, but it would rely on pure faith rather than demonstrable divine intervention and direct contact with divine emissaries.

Likewise, divine spellcasters would still exist, but their magic would be fueled solely by their internal beliefs (you may restrict choices to classes like clerics, who must select domains, or give players complete freedom as to which domains they wish to pursue for their own personal religions); they would no longer be direct servants of supernatural beings.

In such a setting, you would need to decide if resurrection is at all possible and whether there is any place in the setting for the undead. Both of these concepts presuppose something akin to a soul; what happens to a soul in a mundane cosmology? Are souls natural by?products of life, or is a soul some sort of stellar energy adrift in the cosmos? Maybe there is no such thing as a soul, and all who die become ghosts unless some other event transforms them into specific undead creatures.

Where do these ghosts go? How many would remain behind to haunt the site of their death? Do ghosts simply remain invisible and unseen? Do they drift to the edge of the universe?

Would outsiders exist at all? Certainly not in the normal conceptualization. Outsiders in such a cosmology could represent mortal beliefs rather than being an intrinsic part of reality. Powerful spellcasters likely don’t summon them from somewhere else—such outsiders are instead created out of raw magical energy and infused with a specific alignment or characterizing belief. A wizard seeking bloody, destructive revenge might channel his power to create a balor and set the creature loose upon his enemies.

A religious spellcaster might form an angel that embodies her beliefs, rather than believing in the creed of the angel’s divine patron, turning the standard paradigm on its head.

World Tree

The Great Beyond has the River Styx as a planar pathway, whose twisting tributaries weave across the evil-aligned planes and, to a minor extent, beyond. What if there was only one way to travel between planes—not on the Styx, but via a singular transitory plane that touches all other planes known as the World Tree? The World Tree might exist on a Material Plane world, but its trunk extends upward into a sort of haze that connects to the Outer Planes, while its roots burrow unseen into the world below, tapping into the elemental and energy planes.

As the only route between the planes, the World Tree becomes a battleground unlike like any other. Armies of celestials, fiends, inevitables, and proteans seize terrain around the points where the tree’s branches and roots touch other realms in a vast and almost unfathomable war.

Scorched-earth tactics abound; armies of demons march en masse to the gates of Heaven as long as they can manage to physically travel there. The Four Horsemen personally lead their armies down the tree toward the Material Plane, promising their daemons the chance to feast upon all mortal life with nothing but physical distance barring the way. Travel is hazardous, more often than not a question of passing through a war zone as noncombatants struggle to weave between the encamped armies.

What, though, if the World Tree hosted a native race—a species that spawned from the flesh of their great mother tree, perhaps existing in the strange overlapping area somewhere between plant and fey, with their own cities and their own societies that were devoted to protecting the World Tree and regulating travel upon it? Wars would still occur, of course, but with the only path of transit controlled by the outsider race that emerged from its very substance, the World Tree would be less of a war zone and more a neutral ground between the planes. Outsiders and even mortals could travel upon the tree’s branches and up and down its trunk—if their journeys were the will of the World Tree and its chosen servitors. Instead of transit aided by spells and magical components, travelers would offer tribute to the living goddess of the World Tree, hoping for an answer to their prayers in the form of either safe passage or escort.