RuneSlinger 2: Blood Sigils

Two years into his sentence, chaos magician Thor O'Reilly has a reputation, a set of unbreakable principles, and a cellie who keeps him human. What he doesn't have is a plan for Emil Kask — a practitioner twice his age, twice his experience, and completely without ethics — arriving with a contract from Danny Keyes and a message in blood.
  Thor has to win a formal magical duel or stop practicing forever.
  The problem: Kask is stronger. The answer: a practitioner with no principles might have a blind spot Thor can exploit.