Plague of Rats(Complete Divine, p. 174)
Level: Pestilence (CD) 5,
Components: V, S, DF,
Casting Time: 1 round
Range: Medium (100 ft. + 10 ft./level)
Effect: One swarm of rats/two levels, each of which must be adjacent to at least one other swarm
Duration: 1 round/level
Saving Throw: None
Spell Resistance: No
You summon a number of swarms of rats (one per two levels, to a maximum of six swarms at 12th level). The swarms must be summoned so that each one is adjacent to at least one other swarm (that is, the swarms must fill one contiguous area). You may summon the rat swarms so that they share the area of other creatures. Each swarm attacks any creatures occupying its area. The swarms are stationary after being summoned, and won't pursue creatures that flee.
See the Monster Manual for details on rat swarms.