The hunger experienced by all Night Children, or vampires. It’s powerful enough that it seems to create split personalities in some people.
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, and the chosen human must demonstrate great willpower and self-control.
(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.)