A swarm is a dense mass of tiny creatures that alone present little threat but together become deadly. Creature swarms are made up not of normal bats, rats, insects, or other vermin, but of creatures that resemble larger monsters.
“Swarm” is an inherited template that can be added to any creature (referred to hereafter as the base creature) that does not have the swarm subtype. A creature swarm uses all the base creature's statistics and special abilities except as noted here.
Challenge Rating: Same as the base creature +1. If the application of the template removes more than half of the base creature's special attacks, the CR is same of the base creature -1.
Size: The small monsters that make up a swarm come in one of three size categories: Fine, Diminutive, or Tiny. The base creature becomes a swarm of creatures of one of those size categories, according to Table 2-12.
Type: Base creature's type is unchanged, but gains the swarm subtype.
Armor Class: The base creature's size modifier to AC is replaced by the appropriate modifier for the creature swarm's size category. In addition, because the individual creatures in the swarm are smaller than the base creature, the swarm has a lower natural armor bonus than the base creature does (minimum +0). A swarm does not use equipment and does not benefit from armor or shields. The changes to both natural armor bonus and size bonus are summarized on Table 2-12.
Attacks: The swarm loses any melee or ranged attacks that the base creature had. Instead, it gains the swarm attack as specified by the swarm subtype. If the base creature's natural attacks did extra damage, e. g. fire damage, energy drain, etc., the swarm attack does that extra damage as well. If there are multiple attacks with different kinds of extra damage the creature swarm does all the extra damage of all its attacks with its swarm attack.
Space/Reach: A creature swarm occupies a square (if composed of nonflying creatures) or a cube (if composed of flying creatures) 10 feet on a side. Its reach is 0 feet. A swarm does not threaten creatures in its square and cannot make attacks of opportunity with its swarm attack.
Special Attacks: A creature swarm gains the distraction universal monster ability and loses any special attacks that rely on the base creature's size, strength, weapons, combat maneuver bonus, or reach. It retains all the base creature's ranged and area special attacks except those that rely on manufactured weapons.
Though special attacks such as breath weapons, spell-like abilities, or hurled natural weapons (a manticore's spikes, for example) originate from all or most of the creatures that make up the swarm, their parameters (range, area, and so on) are identical to those of the base creature's attack. Damage for all special attacks remains the same as for the base creature.
Spell-like Abilities: A creature swarm keeps all of its spell-like abilities. Those abilities that normally affect a single target affect the creature swarm as a whole.
Spellcasting: A creature swarm keeps all of its native spellcasting abilities. Those spells that normally affect a single target affect the creature swarm as a whole
|Base Creature Size||Swarm Size||Natural AC Decrease||Size Bonus to AC||Str||Dex||Con|
Abilities: The base creature's abilities change as on Table 2-12. Abilities cannot be reduced below 1 by application of this template.
Feats: Same as the base creature, but feats from which the creature swarm can no longer benefit may be replaced with more appropriate choices.
Special Qualities: A creature swarm retains all the base creature's special qualities, gains swarm traits and the additional quality below. The parameters of the special qualities (range, area, and so on) retained from the base creature remain the same.
If the base creature has an Intelligence score of 3 or higher, the creature swarm has a hive mind. So long as it has at least 1 hit point per HD, it retains its Intelligence score, but if its hit points fall below that number, it becomes mindless. As a mindless creature, it becomes immune to mind-affecting effects.
Applying this template requires you to make some decisions about how the base creature would function with the swarm subtype. If you're unfamiliar with that subtype, read its description carefully before starting, and use a swarm creature once or twice during a game before trying to use this template.
Once you have a good grasp of how the swarm subtype works, pick a monster and give this template a try. Carefully consider how each of the base creature's abilities works. In particular, the ability to change shape can be problematic. Do all the creatures in the swarm have to change shape? Do they all take the same shape? If the shapechange would normally affect the creature's abilities, does it affect the swarm's abilities in the same way?