Soul

The Soul of a creature is its essence, the currency of which is Blood.

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The currency of the soul is blood -- once blood is drained from a person, their soul is taken into whatever drained them and can be used as a familiar by Vampires. What is left behind is a Ghoul, an undead agent completely submissive to the will of the vampire that bit it. Even Vampires and other non-human creatures have souls, though Demon souls are considered to be "impure" and "damned". They too are subject to be drained and used as a familiar.