Zarek was born the unwanted son of a Greek slave and a Roman senator. Moments after his birth, his mother gave him to a servant with orders to kill the infant. The servant took mercy on the child and took him to his father who had no more use for the baby than his mother had so Zarek became the whipping boy of a noble Roman family. After he was executed for raping his owner (a crime he didn't commit), he sold his soul to get revenge on the father who treated him wrong and killed the man.
He was isolated in Alaska for centuries because he slaughtered a human village and was considered a danger to anyone around. Astrid was sent to judge him to see whether he was guilty or innocent. In the end she found him innocent mostly because it wasn't humans that he butchered, but Daimons and one Apollite. He and Astrid married and had two children: a son, whom she named Menecaeus but Zarek prefers to call Bob, and a daughter named Kyra.
At the end of Dance with the Devil, it was revealed that Artemis had never taken Zarek's soul because she found him too repulsive to touch. Instead, she fed him ichor to make him immortal and gifted him with the powers and fangs of a Dark-Hunter. Furthermore, Astrid fed him nectar and ambrosia so that he can visit her on Olympus.