Wulf is a Viking who became a Dark-Hunter by getting tricked to switch souls with Morginne, a Dark-Huntress. Unlike the other Dark-Hunters, he was never given an act of vengeance and since he never traded his soul he has different powers than the other Dark-Hunters. The most interesting power and the one that Wulf finds extremely annoying is the fact that no human or animal remembers him five minutes after leaving his presence. The only exceptions to this rule seem to be his blood relatives, who have acted as his Squires over the centuries.
Wulf thought he was doomed to an eternity of not being remembered, since the only ones who remember him were the other Dark-Hunters and Were-Hunters and the only out-clause for a Dark-Hunter is the love of a woman. It's kind of hard to fall in love if nobody remembers him. That is, until he met Cassandra. She remembers him and she accepts him. There's just one little problem. She's a half-Apollite whose time will run out in eight months. She's also the last direct descendant of the god Apollo. If she dies without an heir, the world ends.