A swarm is a collection of Fine, Diminutive, or Tiny creatures (and sometimes even small, medium or large 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. A swarm 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.
A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking. Assuming a rogue can qualify for sneak attacks against a swarm, their sneak attacks still do half damage, because rogues cheat.
Swarms are immune to any attacks which target the swarm's Maneuver Defense.
Swarms take only half damage from slashing and piercing weapons and full damage from bludgeoning weapons regardless of the component creature size. Furthermore, weapons with a magical property which adds elemental damage of some kind (e.g. flaming, shocking burst, etc.) always do full damage to swarms.
Reducing a swarm to 0 hit points or less 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, morale effects, patterns, and phantasms) 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 those created by gust of wind. 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.
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 stat block has "swarm" in the Melee entries, with no attack bonus given.
Swarms typically only deal half the listed damage for their CR. This is to compensate for the auto-hit nature of their attacks.
A swarm's attacks are non-magical, unless the swarm's description states otherwise. Damage reduction sufficient to reduce a swarm attack's damage to 0, being incorporeal, or other Special Abilities usually give a creature immunity (or at least resistance) to damage from a swarm. Some swarms also have acid, blood drain, poison, or other special attacks in addition to normal damage.
Swarms do not threaten creatures, and do not make attacks of opportunity with their swarm attack. However, they distract foes whose squares they occupy, as described below.
Swarm Distraction Ability
Swarms possess the distraction ability:
- Any living creature that takes damage from a creature with the distraction ability must make an Average DC Fortitude save, or become Afflicted. If the creature makes the save, they are instead Sickened. If they take damage on any following round while still Sickened or Afflicted, they must make another Average DC fortitude save, or move up one on the Status Array. While sharing a space with the swarm, you cannot get rid of the Sickened condition. At the end of any round in which you are outside of the swarm, you may make another Average DC Fortitude save to attempt to move down a step on the status array. (While outside of the swarm's space, you cannot go up on the status array if you fail the save, you simply remain at the status you currently have.)
- In addition, spellcasting or concentrating on spells within the area of a swarm requires a Caster Check vs the monster Maneuver Defense. Using skills that involve patience and concentration requires a Hard DC Will save.
Multiple swarms can move into, stop and occupy spaces that other swarms occupy. However, no more than two swarms can inflict damage or effects on a single target.