The Vampire Bat is a Mob exclusive to the Hamlet DLC. Vampire Bats are essentially the Hamlet version of Hounds : They will attack the player periodically and will come in waves where their numbers will increase the more days the player has lived. They have 130 health and can be kited in the same way as Hounds.
Apart from their periodic attacks, they can also be found sleeping in their Den, a Cave Cleft. The Vampire Bat will seemingly stay asleep until something violent wakes it up, as a player's attack or an earthquake.
The player can tell when an attack is about to happen when swooping shadows of bats overhead appear, very similar to Buzzards in Reign of Giants or Peep Hens in Hamlet. They usually happen every 5 days, but its mostly randomized
Vampire Bats are very hard to deal with during Humid Season, as the player's visibility is greatly reduced, making kiting hard.