Azazel, also known as the Yellow Eyed Demon, The Demon, and The Devil, was the main antagonist. Azazel's name was not revealed and he was referred to by nicknames such as "The Demon", "The Yellow-Eyed Demon", or "Yellow Eyes." In this version, Azazel was originally an Angel before becoming a Demon, while in the live action TV series, he was presumably a Human soul that was tortured and corrupted in Hell as other Demons were. In this version it would seem that Azazel was also the devil however it can not be confirmed though when called the Devil he never denied it. Once he killed Mary Winchester, the Winchester men hunted him for nearly two and a half decades.Azazel fed his blood to an infant Sam Winchester, transforming him into a Special Child. Azazel was killed with The Colt by Dean Winchester in revenge in the finale; however, his actions continued to influence events and characters even years after his death. For example, both Sam and Dean have been haunted by visions or hallucinations of him. (Source: Supernatural Wikia)