Swarm of Undead Rats
Wherever powerful undead creatures lurk, a single dead rat might be reanimated by the ambient necrotic energy in the area. Thus warped and twisted, the undead rat can overtake its living counterparts and rapidly multiply into an infestation of hundreds or thousands. No longer requiring air to breath, swarms of undead rats can be found in shallow water ways such as sewers, swamps and bogs, where they will lie silent and unmoving until disturbed by a hapless adventurer not watching their step. Swarms will then attack in groups, overhwelming individual targets.