Azazel was a powerful demon who served Lucifer. Until Sin City, Azazel's name was not revealed and he was referred to by nicknames such as "The Demon", "The Yellow-Eyed Demon", or "Yellow Eyes". Once he killed Mary Winchester, the Winchester men hunted him for nearly two and a half decades and finally killed him in revenge in the second season 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.
Okay, Azazel's past is pretty shady. When it comes to his human life or how he even came to come across the vessel that he uses to terrorize. What we do know is that this yellow eyed demon climed up the ranks in hell and became one of Lucifer's most devoted and trusted servants, to the point where he ruled hell following his "father's" imprisonment. Eventually, he took charge of the plan to find Lucifer and free him from his cage. He also took on a "daughter" Meg, a student of Hell's Grand Torturer Alastair. He also claimed the demon Tom as his "son." He was close enough to Lucifer to know of the location of Lucifer's Crypts and toured them all with Meg at some point. At some point, Azazel left Hell and went to Earth to search for the entrance to Lucifer's cage. As time goes on Azazel does what demons do best, make deals. He came across Sam and Dean's mother and cut a deal with her. In 1983, Mary discovers Azazel standing over her baby Sam's crib, feeding his blood to the infant. Upon being interrupted, the demon pins her to the ceiling, slashes her stomach and causes her to burst into flames. Mary's death inspires John to dedicate his life to hunting down Azazel, at the same time training Sam and Dean to hunt supernatural creatures. Perhaps because of the enormity of his task (of selecting and grooming potential vessels for Lucifer across the world), Azazel rarely encountered the Winchesters directly; instead, he sent his children to confront them and manipulate Sam in his stead.
This demon was a pain in the ass. Cunning and as Dean would "son of bitch" . I did not dig too deep into this demon before I spill too many spoilers!
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