A Venefica was a Roman sorceress who used drugs, potions, and poison for several reasons.
Later it appears in one of Sir Francis Bacon's Essays,"Of Friendship" in the following lines: "And it seemeth his favour was so great, as Antonius, in a letter which is recited verbatim in one of Cicero’s Philippics, calleth him ‘venefica,’ witch, —as if he had enchanted Caesar."
In this novel, veneficas are defined as "girl slaves who are, from their infancy, fed certain poisons, first in minute amounts, then in increasing doses throughout their upbringing.
By the time they are grown to maidenhood, their own bodies are accustomed to these substances and are unharmed by them.
However, so virulent is the accumulated poison, that a man who beds with a venefica - or anyone who partakes of any of her juices - dies on the instant."