Ephemeral swarm

Ephemeral swarms were large groups of ghostly undead that had risen from the psychic agony and anguish of many small creatures suffering a common, violent death.

Description
Ephemeral swarms varied widely in appearance, always corresponding to whatever group of animals had given rise to it. For example, an area in which cats died off from a plague or a nest of rats killed by a fireball would result in an ephemeral swarm of cats and rats respectively.

Behavior
All ephemeral swarms sought to inflict upon others pain and suffering, just as they had endured in the moment of their demise. They would especially focus their attention towards those they could recognize as being an agent of their death.

Abilities
These swarms of creatures had incorporeal bodies, floated above the ground, and could see in areas of total darkness. In addition, their bites carried with them negative energy, causing those they bit to slowly have their vitality drained away.

Habitats
Ephemeral swarms existed partially on the ethereal plane. On the Prime Material plane, they were typically found haunting the Underdark, various dungeons across Faerûn, and the labyrinthian sewers of some of Faerûn's oldest cities. Once such city was Skuld, the capital of Mulhorand, which had quite ancient ephemeral cat swarms haunting its sewers.