Just an FYI, on the 2nd level of the 6th floor these guys each spawn "an enraged vampire" upon it's death. lvl 41 rogue spawns each time you kill a bat.
on the 7th floor, only, these have a chance to spawn Tserrina.
As asselinmatt posted, on the 6th floor, these also spawn "an enraged vampire". But, on the 6th floor, only enraged vampires spawn upon the death of these bats.
-- Ryllian, 70 Elf Ranger of Povar (formerly Quellious)