A swarm of little creatures carpets the effect’s area, viciously attacking all other creatures there. (Roll on the table below to see what sort of creature is summoned.) A creature in the swarm who takes no actions other than fighting off the creatures takes 1 point of damage on its turn. A creature in the swarm who takes any other action, including leaving the swarm, takes 1d4 points of damage +1 point per three caster levels. Spellcasting or concentrating on spells within the swarm is impossible.

1d20	Swarm Type*
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1–8 	Rats (animals)
9–14 	Bats (animals)
15–16 	Spiders (vermin, poisonous)
17–18 	Centipedes (vermin, poisonous)
19–20 	Flying beetles (vermin)

*The creature types (in parentheses) indicate what sorts of spells and effects might aid a subject against the swarm.

A swarm of poisonous creatures deals no damage to creatures who are immune to poison, though it still prevents spellcasting and concentration. The creatures’ attacks are nonmagical. Damage reduction, being incorporeal, and other special abilities also make a creature immune to damage from the swarm.

The swarm cannot be fought effectively with weapons, but fire and damaging area effects can force it to disperse. The swarm disperses when it has taken a total of 2 hit points of damage per caster level from these attacks. Certain area or effect spells, such as gust of wind and stinking cloud, disperse a swarm immediately if appropriate to the swarm summoned. (For example, only fliers are affected by a gust of wind.)

The swarm is stationary once summoned. A druid caster, however, can (as a move-equivalent action) direct the swarm to move up to 30 feet per round (or 90 feet per round if she has summoned bats or beetles).