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Deep One

Deep One CR 2

XP 600
CE Medium humanoid (aquatic, deep one)
Init +3; Senses darkvision 60 ft., low-light vision; Perception +9


AC 14, touch 9, flat-footed 14 (–1 Dex, +5 natural)
hp 22 (3d8+9)
Fort +6, Ref +0, Will +6
Immune pressure damage; Resist cold 5


Speed 20 ft., swim 40 ft.
Melee 2 claws +5 (1d4+3)


Str 17, Dex 9, Con 16, Int 14, Wis 17, Cha 12
Base Atk +2; CMB +5; CMD 14
Feats Great Fortitude, Improved Initiative
Skills Knowledge (religion) +8, Perception +9, Stealth +5 (+9 underwater), Swim +17, Use Magic Device +7; Racial Modifiers +4 Stealth underwater
Languages Common, Deep One
SQ amphibious, deep one traits, fecund nature, item use


Deep One Traits (Ex)

All creatures with the deep one subtype gain the following shared traits.

Deep Dweller (Ex)

Deep ones are immune to damage from water pressure. Their bodies are capable of instantly adjusting to different water depths or even the surface with ease.

Deep One Skills (Ex)

Deep ones are fanatically religious, typically in the worship of Cthulhu or one of their own transcended ancestors like Father Dagon or Mother Hydra. A deep one gains one bonus skill rank per Hit Die that must be placed in Knowledge (religion), and this skill is a class skill for all deep ones. If a creature gains deep one traits as a result of going through the change, these skill ranks are added retroactively. If the creature already possesses skill ranks in Knowledge (religion), it does not gain these additional skill ranks.

In addition, Perception, Stealth, Swim, and Use Magic Device are class skills for deep ones.

Fecund Nature (Ex)

A deep one can interbreed with any living vertebrate creature. Creatures born as a result of this interbreeding resemble the species of the non-deep one parent but gain the deep one hybrid template.

Immortal (Ex)

A deep one does not age. Barring death from violence, disease, or misadventure, a deep one can live forever. Deep ones are immune to the effects of magical aging.

Item Use (Su)

A deep one can activate spell-trigger items like staves and wands as if it were a spellcaster of the appropriate class.

Resist Cold (Ex)

A deep one is unharmed by the negative effects of cold temperatures (such as those found at the bottom of oceans), and has cold resistance 5.

Senses (Ex)

A deep one has darkvision 60 feet and low-light vision.

Take to the Water (Ex)

A deep one has the aquatic subtype and the amphibious universal monster ability.


Environment any water
Organization solitary, gang (2–5), cult (6–12), or shoal (13 or more)
Treasure standard

Deep Ones Culture

The deep ones are an incredibly strange species.

Though they live undersea, they are capable of operating on land. They are unusually fecund and can interbreed with other species, producing (eventually) more deep ones rather than hybrids. They are able to grow (or shrink) to different sizes, based on diet. They have their own culture, ancient and independent, yet integrally tied to and invested in humanity and other mortal cultures.

The key to the deep ones’ strange nature is that they are an artificial species, created by Great Cthulhu to interact with and possibly supplant humanoids. Although not an ancient race compared to many in the Mythos, their culture is older than those of most humanoids. Physically, they are roughly similar to humanoids (in that they are bilaterally symmetrical and have four limbs), so that they can interact more effectively. They use vocal sounds for speech.

Deep Ones Lifecycle

The deep ones are able to reproduce with almost any other species of appropriate size. Initially, the resulting young look like the non-deep one parent. Thus, if a deep one mates with a shark, the spawn looks like a shark. If it mates with an elf, the spawn looks like a baby elf.

Over time, as the spawn grows and matures, eventually it will begin to make the transformation into a deep one. In humanoid races, such as orcs, humans, or dwarfs, this starts usually a decade or two after the person reaches physical maturity and takes a significant amount of time to complete. In humans, the hybrids would usually be in their 30s before the metamorphosis begins, and in another decade they resemble deep ones. Psychic or emotional trauma or shock can accelerate or stimulate the change.

The more different the mate is physically from the deep one (particularly in terms of bone structure), the longer the metamorphosis takes. For example, a dolphin may take two or three times as long to complete the change as a human.

Once the deep one change is complete, the entity typically joins the rest of the deep ones beneath the waves. Ideally, the deep ones are aware of their spawn and keep in contact with them, even putting together social constructs to educate and control such individuals (such as the Esoteric Order of Dagon).

Deep Ones Malformation and Error

The transformation process does not always go well.

Some hybrids never manage to complete the change and stall partly into the process.

Other genetic errors can occur, and sometimes disease or heredity affects the change for the worse.

These malformed hybrids rarely gain a formal position within the deep one society but instead tend to be hangers-on.

The deep ones or more fortunate hybrids sometimes use them as tools or pawns.

Deep Ones Under the Sea

Deep ones are generally one of the most socially and magically sophisticated races of the sea, in their own unsettling way. They typically construct large and complex cities deep under the waves. When these are located near a shore, the local population is often subverted by the deep ones in many ways.

Deep one society is stratified and controlled by its older, higher ranking members. One feature of deep one biology is that, if fed sufficiently, they continue to grow to indefinite size. Their leaders ensure that this only proceeds under their control: only those deep ones that are needed to be huge in size are permitted to grow unhindered. On occasion, there have been entire cities of gigantic deep ones, but in a typical city, only a few individuals exceed the normal size because a large population of huge deep ones requires more food than most cities can support. Also, giant deep ones cannot breed easily with the mortal races.

When deep ones grow to Large size, they do not necessarily stay humanoid in appearance. Their body form can continue to mutate and adapt, and so extremely large individuals may look very different from their smaller kin (for instance, Dagon and Hydra). The deep ones have great skill in biology and genetics, and thus can often control and direct such growth to suit their nefarious purposes.

Deep ones are also capable of domesticating and using other types of undersea creatures, particularly Mythos beings such as shoggoths.

Interaction with Deep Ones

The deep ones maintain constant surveillance and contact with the surface world near their homes.

Their ultimate purpose is not their prerogative, but determined by Cthulhu’s design. They not only monitor mortal activity for the Great Old One, but may someday be used to supplant the mortal realm.

Their hybridization and inbreeding is one important tool toward that goal, for it allows them to blend in with mortals.

The hybrids, of course, knowing they will one day be full deep ones, are fully supportive of the deep ones’ goals.

Deep ones are known to have originated many magic items, including the medallion of Y’ha Nthlei, seal of Hydra, starstone of Mnar, and tiara of Mnar. They also practice a profoundly dangerous method of creating twshas to control shoggoths and proto-shoggoths.

Deep Ones Religion

The most consistent quality among Mythos religions is that they are, at their core, materialistic. Cthulhu is worshiped not for his greatness and glory, but because when he returns to power, his worshipers will be able to kill and take joy as they please. The King in Yellow provides his cultists with wisdom and emotions beyond the ken of normal folk. Nyarlathotep inspires endless indulgence and appeals to cultists’ greed and base desires.

Nowhere is this more obvious than among the aberrations and outsiders who serve the Great Old Ones and Outer Gods. In almost every case, they worship these powerful entities because they were made to or because they receive specific blessings from them.

Almost all Great Old Ones and Outer Gods are crassly materialistic themselves in terms of the rewards they bestow upon worshipers. No heaven or afterlife is offered, but rather treasure, magical power, and wisdom. Perhaps the Great Old Ones envy more conventional gods, who are served with true love and self-sacrificing service. Who can say?

Section 15: Copyright Notice

Sandy Petersen’s Cthulhu Mythos, © 2017, Petersen Games; Authors: Sandy Petersen, Arthur Petersen, Ian Starcher.