The hunger experienced by all Night Children, or vampires. It’s powerful enough that it seems to create split personalities in some people.
Notte has never quite been clear on whether it’s actually another living consciousness or not. He IS clear, however, that it can be controlled, and must be before he’ll allow his children out into the world.
A newly-made vampire is always overwhelmed by the Beast, losing their minds to the hunger. It’s one of the reasons Notte is so careful about who is “made” and when; facilities to contain and feed this new Night Child must be readily available.
Most new vampires gain control of it and regain their senses within the first year.
Some take longer.
A sad few never have at all, and Notte keeps them carefully asleep, away from the world.
(Note: Ravena is not so careful with her own children, but she relishes the lack of control. To her, lack of control is a sign of strength – but that’s a whole other story.)