The Enemy Within — Christie Golden

Sir Tristan Hiregaard is a nobleman in the dark land of Nova Vaasa, to all outward appearances a kind lord who would never harm anyone. Yet, Sir Tristan has a sinister secret, one that even he does not understand... .

At times the nobleman transforms into a brutish creature named Malken, a man-beast who finds no act too base, just as long as it extends the killing grip of his vast criminal empire. No one is safe from Malken, and it seems that no one can break his stranglehold on Nova Vaasa--until Tristan himself takes on a quest to destroy his evil side.