Ever wonder what would happen if you took a giant bat, a dragon, a zombie, and put them all in a blender? If you ever did, then behold! The Terrorgheist (known colloquially as names like “Fluffy” to their vampiric masters) is a massive, batlike undead dragon monster ridden exclusively by Strigoi/Ghoul Kings. Their exact nature is different depending on which flavour of Warhammer; in Warhammer Fantasy, they're oversized bats that have been fed vampire blood and bound to their master after death, whilst in AoS they’re all long dead monsters brought back to life via Shyishian sorcery and viewed by their deluded masters as noble winged beasts.
Terrorgheists attack mainly through both their piercing shriek; one of the many things you don’t want to hear as some average schmuck in an Empire or Free City army is the wail of a Fluffy, because it almost always means that you are about to get fucking devoured. And that’s not the worst part; if you somehow manage to bring this batty bastard down, it will explode into a swarm of much smaller bats that’ll claw out your damn eyes. Lovely.