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)
There are no shrouded bats on the 4th floor. If you indeed have killed one on the 4th floor, someone charmed it on the 6b or the 7th floor and successfully parked it on 4th floor before they logged out.