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.
“Creature swarm” is an inherited template that can be added to any corporeal creature (referred to hereafter as the base creature). A creature swarm uses all the base creature’s statistics and special abilities except as noted here.
Challenge Rating: The creature swarm’s CR is the same as the base creature’s unless the application of the template removed many of its special attacks. Consider also how the swarm subtype affects combat, and pay special attention to the fact that the swarm subtype grants immunity or half damage from weapon attacks. Such considerations might be grounds for increasing the creature swarm’s CR.
Size and Type: Type is unchanged, but the creature gains the swarm subtype. The small monsters that make up a swarm come in one of three size categories: Fine, Diminutive, or Tiny. Thus, the base creature becomes a swarm of creatures of one of those size categories, according to the following table.
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 the following table.
Base Attack: The creature swarm retains the base attack bonus of the base creature, but it cannot grapple (see the description of the swarm subtype).
Attack and Full Attack: The swarm loses any melee or ranged attacks that the base creature has because creatures with the swarm subtype don’t make standard melee or ranged attacks. Instead, they deal automatic damage to any creature whose space they occupy at the end of their move (no attack roll needed). Such swarm attacks are not subject to a miss chance for concealment or cover.
Damage: A creature swarm deals damage according to its Hit Dice, as given on the following table. Strength modifiers do not apply to this damage.
Incorporeality or damage reduction sufficient to reduce a swarm attack’s damage to 0 may give a creature immunity to damage from a creature swarm. If the base creature’s natural or unarmed attacks are considered aligned or magical, or have some other quality that can overcome damage reduction, the damage for the swarm attack has the same effect. If the base creature’s natural attacks also deal energy damage, ability damage, poison, disease, paralysis, or some other special effect, the damage for the swarm attack has the same effect, and the damage (if any) is the same.
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 loses any special attacks that rely on the base creature’s size, strength, weapons, or reach, including grappling, rending, bull rushes, overruns, or trampling. It retains all the base creature’s ranged and area special attacks except those that rely on manufactured weapons, and it also gains the one described here. 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.
Distraction (Ex): Any living creature vulnerable to the creature swarm’s damage that begins its turn with a creature swarm in its square is nauseated for 1 round. A Fortitude saving throw (DC 10 + 1/2 creature swarm’s HD + creature swarm’s Con modifier) negates the effect. Even with a successful save, spellcasting or concentrating on spells within the area of a creature swarm requires a Concentration check (DC 20 + spell level). Using skills requiring patience and concentration requires a Will save (DC 20).
Special Qualities: A creature swarm retains all the base creature’s special qualities and gains those described here. The parameters of the special qualities (range, area, and so on) retained from the base creature remain the same.
Hive Mind (Ex): 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.
Swarm Traits: A creature swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking. A creature swarm made up of Tiny creatures takes half damage from slashing and piercing weapons, and one composed of Fine or Diminutive creatures is immune to all weapon damage. Reducing a creature swarm to 0 or fewer hit points causes it to break up, though damage taken until that point does not degrade its ability to attack or resist attack. Creature swarms are never staggered or reduced to a dying state by damage. Also, they cannot be tripped, grappled, or bull rushed, and they cannot grapple another. A creature swarm is immune to any spell or effect that targets a specific number of creatures (including single target spells such as disintegrate), with the exception of mind-affecting effects if the swarm has an Intelligence score and a hive mind. A creature swarm takes a –10 penalty on saving throws against spells or effects that affect an area, such as many evocation spells or grenade-like weapons. If the area attack does not allow a saving throw, the swarm takes double damage instead. Creature swarms made up of Diminutive or Fine creatures are susceptible to high winds, such as those created by a gust of wind spell. For the purpose of determining the effects of wind on a creature swarm, treat it as a creature of the same size as its constituent creatures. Wind effects deal 1d6 points of nonlethal damage to the swarm per spell level (or Hit Die of the originating creature, in the case of effects such as an air elemental’s whirlwind). A swarm rendered unconsciously nonlethal damage becomes disorganized and dispersed. It does not reform until its hit points exceed its nonlethal damage.
Abilities: Change from the base creature according to the following table.
Feats: Same as the base creature, but feats from which the creature swarm can no longer benefit may be replaced with more appropriate choices.
Treasure: None. Creatures with the swarm subtype do not use equipment.