SRD:Swarm Subtype

Swarm Subtype
A swarm is a collection of Fine, Diminutive, or Tiny creatures that acts as a single creature. A swarm has the characteristics of its type, except as noted here. A swarm has a single pool of Hit Dice and hit points, a single initiative modifier, a single speed, and a single Armor Class. A swarm makes saving throws as a single creature. A single swarm occupies a square (if it is made up of nonflying creatures) or a cube (of flying creatures) 10 feet on a side, but its reach is 0 feet, like its component creatures. In order to attack, it moves into an opponent’s space, which provokes an attack of opportunity. It can occupy the same space as a creature of any size, since it crawls all over its prey. A swarm can move through squares occupied by enemies and vice versa without impediment, although the swarm provokes an attack of opportunity if it does so. A swarm can move through cracks or holes large enough for its component creatures.

A swarm of Tiny creatures consists of 300 nonflying creatures or 1,000 flying creatures. A swarm of Diminutive creatures consists of 1,500 nonflying creatures or 5,000 flying creatures. A swarm of Fine creatures consists of 10,000 creatures, whether they are flying or not. Swarms of nonflying creatures include many more creatures than could normally fit in a 10-foot square based on their normal space, because creatures in a swarm are packed tightly together and generally crawl over each other and their prey when moving or attacking. Larger swarms are represented by multiples of single swarms. The area occupied by a large swarm is completely shapeable, though the swarm usually remains in contiguous squares.

Traits
A swarm has no clear front or back and no discernable anatomy, so it is not subject to critical hits or flanking. A swarm made up of Tiny creatures takes half damage from slashing and piercing weapons. A swarm composed of Fine or Diminutive creatures is immune to all weapon damage. Reducing a swarm to 0 hit points or lower causes it to break up, though damage taken until that point does not degrade its ability to attack or resist attack. 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 an opponent.

A 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 (charms, compulsions, phantasms, patterns, and morale effects) if the swarm has an Intelligence score and a hive mind. A swarm takes half again as much damage (+50%) from spells or effects that affect an area, such as splash weapons and many evocation spells.

Swarms made up of Diminutive or Fine creatures are susceptible to high winds such as that created by a gust of wind spell. For purposes of determining the effects of wind on a swarm, treat the swarm as a creature of the same size as its constituent creatures. A swarm rendered unconscious by means of nonlethal damage becomes disorganized and dispersed, and does not reform until its hit points exceed its nonlethal damage.

Swarm Attack
Creatures with the swarm subtype don’t make standard melee attacks. Instead, they deal automatic damage to any creature whose space they occupy at the end of their move, with no attack roll needed. Swarm attacks are not subject to a miss chance for concealment or cover. A swarm’s statistics block has “swarm” in the Attack and Full Attack entries, with no attack bonus given. The amount of damage a swarm deals is based on its Hit Dice, as shown below.

A swarm’s attacks are nonmagical, unless the swarm’s description states otherwise. Damage reduction sufficient to reduce a swarm attack’s damage to 0, being incorporeal, and other special abilities usually give a creature immunity (or at least resistance) to damage from a swarm. Some swarms also have acid, poison, blood drain, or other special attacks in addition to normal damage.

Swarms do not threaten creatures in their square, and do not make attacks of opportunity with their swarm attack. However, they distract foes whose squares they occupy, as described below.

Distraction (Ex)
Any living creature vulnerable to a swarm’s damage that begins its turn with a swarm in its square is nauseated for 1 round; a Fortitude save (DC 10 + 1/2 swarm’s HD + swarm’s Con modifier; the exact DC is given in a swarm’s description) negates the effect. Spellcasting or concentrating on spells within the area of a swarm requires a Concentration check (DC 20 + spell level). Using skills that involve patience and concentration requires a DC 20 Concentration check.