"I encountered the most troublesome bats in the caves outside Westmarch. My companions and I had sheltered there with the intent of organizing my many records, when the miserable creatures appeared and tore several of the scrolls to pieces! Truly, the world shall be a poorer place for this lost wisdom."— Abd al-Hazir(src)
Vile Bats are smaller than Carrion Bats, but are also much faster. They do not have any elemental damage added to their attack, and are mostly found in caves beneath the Blood Marsh and Paths of the Drowned. Usually Vile Bats attack in large numbers. They also appear in the Tidal Caves beneath the Greyhollow Island, and (for a single event, The Forest Prayer Disturbed) on the island itself.
Three to four Vile Bats will also sometimes spawn when a Corpse Raiser is slain. These bats are spectral green in color and transparent as if they were ghosts.