Vampiric Impulse

The base desire possessed by both True Ancestors and Dead Apostles.
In True Ancestors it is merely a bad habit, but in Dead Apostles it is a means of staying alive.
For those Dead Apostles who used to be human, it is something not entirely dissimilar from an expression of love.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary

Even the species akin to the gods that are True Ancestors is burdened with a flaw since birth. Regardless of it not being necessary for their vital activity, they seek “human blood” due to a primeval desire to want to absorb information from intelligent life forms ——— This is an instinct as a species called Vampiric Impulses.
The True Ancestors who kept sucking blood and indulging in pleasure, would eventually become powerful vampires and begin the pollution of the world. They would fall to become beasts.
In order to prevent that disgraceful fate, they used their powerful superhuman abilities to suppress their Vampiric Impulses.
When they realize they cannot hold their urges any longer, True Ancestors will choose eternal sleep for themselves. That is equivalent to being a life span for those who naturally don’t have a life span, and the end of their lengthy journey.

Tsukihime Material I: Tsukihime Work Glossary