Azazel (Supernatural)

"I have plans, Sammy, plans for you and all the children like you..."

- Azazel to Sam Winchester, while possessing Sam's father John.

Azazel, also known as the Yellow-Eyed Demon, is the tyrannical ruler of the demons in the TV series Supernatural, and the main antagonist for the first two seasons.

In 1972, after searching for centuries, he found the opening above his fathers cage, he killed several nuns in a convent to speak to his father, Lucifer, who was imprisoned in the deepest circle of Hell, Lucifer's Cage. Lucifer told him he must release Lilith from Hell by opening the Hell's Gate.

So Azazel began making deals with people. Unlike most demons, Azazel's deals did not involve the person's soul; instead what he received was permission to return later without that person trying to stop him. His intent was to bleed into the kids' mouths and give them supernatural powers, so that he could awaken a dark side inside them and get them to open the Hell's Gate, since it was set in the middle of a railroad shaped like a Devil's Trap (which traps any demon that enters it).

One person who made a deal with him was Mary Winchester. He killed her boyfriend John, along with her parents, so she made a deal with him to bring John back to life. When their second son, Sam, was six months old, Azazel entered to bleed into Sam's mouth. Mary, previously unaware of what Azazel intended to do, tried to stop him, so he burned her to death. This event turned the entire Winchester Clan into hunters.

Later, he got Sam back into the hunting business with his brother Dean, when he sent the demon Brady to kill Sam's girlfriend Jessica. Sam, Dean, and (separately) John hunted him down. After they began working together, the three of them found a gun called the Colt, which can kill anything, even a demon (as regular weapons or methods of killing will not work on demons). Eventually Azazel possessed John, but he was able to take control due to Azazel being shot in the leg with the Colt, and tell Sam to shoot him. The colt, at the time, had one bullet in it. Dean told Sam not to shoot him, as that would kill their father as well. Azazel was eventually able to leave John and send a demon to injure them with a truck.

Sam and John survived, but Dean was mortally wounded. To save his son John made a deal with Azazel to heal Dean, in exchange for the Colt and his soul. So Azazel possessed the Reaper, Tessa, and healed Dean. Then he killed John, sending his soul to Hell.

Eventually, Azazel put the final phase of his plan into motion and gathered all the surviving psychics he created, including Sam, at a ghost town, where their object was to kill each other, as Azazel only needed one. Jake was the ultimate survivor, killing Sam. Azazel gave him the Colt and told him to open the Hell's Gate. However, Dean made a deal with a demon to revive Sam, and together they went to the cemetery that the Hell's Gate was located in. After Jake opened the Hell's Gate and was killed by Sam, Azazel trapped Dean and was about to kill him with the Colt's last bullet when John's spirit escaped through the Hell's Gate and pulled Azazel out of his human body. By the time Azazel took repossession of it, Dean had picked up the Colt, and before Azazel could do anything to stop him, Dean shot him in the heart with the Colt, killing him.

Despite his death, however, he had successfully opened the Hell's Gate and released Lilith, thus allowing the next phase of the demons' plan to release Lucifer to eventually go into effect.

His name was not revealed until some time after his death, mentioned by a demon in Season 3. Sam and Dean always called him "the yellow-eyed demon", "the demon", or "Yellow-Eyes", and continued to do so even after learning his name.

He seemingly returned at the beginning of the sixth season, saying God brought him back, but this turned out to be a hallucination caused by a trio of Djinn.

Powers and Abilities
Azazel was one of the most strongest demon in existance (only rivaled by Lilith that later appeared in Season 3), able to create and manipulate fire, move objects or people with his mind, travel from place to place by thought, enter dreams, make electrical devices go haywire, ressurect, heal, and warp reality through a deal, and cause internal injuries by thought.

He was also stronger than all other demons, and immune to almost all demonic weaknesses.

It is unclear whether Ruby's knife has any effect on him, as Alastair is shown to be resistant to it and the knife was not introduced until the episode after Azazel's death.

Trivia

 * Azazel is the only known yellow-eyed demon that featured in series. It's unclear whether any other yellow-eyed demon do exist aside him.