Frederic is haunted or hunted by ... something. Throughout his life, dark riders have descended on him every year around his birthday. Although they have not yet harmed him directly, they rampage around him killing and destroying. He thinks they are demonic in origin and his original mistress, Katerina of Verditius, constructed various weapons and defenses on that theory.
Although they tools that Katerina and Frederic made seemed effective, Katerina was murdered and the tools stolen or destroyed forcing Frederic to seek sanctuary with the magi of Lakehome where he was adopted by Terrus.