Vampires are Dark Ka creatures created by the Selenim named Arukashu. Like all descendants of the Selenim, vampires must consume Solar Ka in order to sustain themselves. In their specific case, by consuming blood.
Arukashu created the first vampires by instilling in select humans a fragment of his essence. In turn, those vampires could do the same to other to create more vampires. Each creation weakens the the creator slightly, and each generation of vampire is weaker than the last. Most vampires below the fifth generation are not capable of creating more vampires. For the above reasons, the number of vampires in existence at any given time is rather small.