Summon Swarm

Warlock Invocation
Level: Least, 2nd lv.
Components: V, S, M
Casting Time: 1 standard action
Range: close (25 ft. + 5 ft./2 levels)
Duration: Concentration+2
Saving Throw: None
Spell Resistance: No

You summon a swarm of bats, rats, or spiders (your choice), which attacks all other creatures within its area. (You may summon the swarm so that it shares the area of other creatures.) If no living creatures are within its area, the swarm attacks or pursues the nearest creature as best it can. The caster has no control over its target or direction of travel.

Warlock Invocation:

You can use summon swarm as the spell with this invocation. Unlike the spell, this invocation has a duration of concentration instead of concentration + 2 rounds.

(Complete Arcane, Page 134)