Zamorak, (often nicknamed Zammy by players), is one of the three Gods of Gielinor, representing Chaos and Anarchy.
He is the source of many taints of "evil" (in the opinion of most people), especially in human nature. His followers were responsible for the burning of the original Wizards' Tower, causing the loss of libraries' research. Zamorak was the second main participant in the God Wars and thus corrupted many parts of the would-be orderly world.
As with Zamorak being the main god of chaos does not necessarily make him evil, it's usually his opponents (such as Saradominists) who claim him to be.
Several clans have centered their morales and diciplines around Zamorak.