Abaddon

Abaddon is a Knight-of-Hell, a very powerful demon that hunted Henry Winchester and a mysterious supernatural object in possession of his order. Abaddon occupied a Josie Sands, a woman acquainted with the Order-of-Letters.

Abaddon is single-handedly responsible for the extinction of the Men-of-Letters.

Personality
Like most demons, Abaddon was sadistic, treacherous and power hungry. She enjoyed causing suffering to others, enjoyed mocking them, and basked in their fear of her. Despite her sadism, she rarely prolonged out the torment of her victims, preferring fast and brutal deaths. She was also greedy, and very cunning.

Abaddon was also quick to anger, and enjoyed flaunting her superiority to others. However, her arrogance did cause her to underestimate and underplay her opponents, which, in the end proved to be her downfall.

Powers and Abilities
Although she is a black-eyed demon, Abaddon is also a knight-of-hell and thus far more powerful. She displayed unique abilities not seen even from higher-ranking demons. Her screams alone could trigger lightning and thunder.
 * Telekinesis - The ability to move people and objects with one's mind. Abaddon was capable of flinging Sam and Dean across a large warehouse with ease.
 * Demonic Possession - Like every other demon, Abaddon can only become corporeal by possession of a human's body to be active on Earth.
 * Biokinesis - Abaddon caused Larry Genem' eyes to bleed and caused him to go blind.
 * Pyrokinesis - Abaddon caused candle lights to burn out. She also burned the headquarters of the Order-of-Letters with fire as explained by Henry Winchester.
 * Super Strength - Abaddon easily overpowered a group of men-of-letters elders. In addition, she could also easily snap a human's neck with just one hand, or slit the throat of a human with just her nails.
 * Spell Casting - Abaddon was able to cast a spell to travel through time to follow Henry into the future.
 * Immunity - Abaddon was unaffected by the exorcism rite performed by at least one in a group of men-of-letters elders. She was also able to resist the effects of the demon-killing knife.
 * Memory Reading - By inserting part of her essence into a human, she can access the memory of the target.

Weaknesses

 * Devil's Trap - Like all other demons, she was unable to escape a Devil's Trap.
 * Archangels - The eldest and most powerful of the angels can destroy her kind.
 * Demon-killing knife -  The demon killing knife was able to hurt Abaddon but couldn't kill her.

Early Life
Abaddon was one of the first fallen, handpicked by Lucifer himself to become one of the first demons. She is possibly the last surviving member of the Knights of Hell, as the Archangels were thought to have killed them all.

In John Winchester's journal it is said that he had tortured a demon that had mentioned working for Abaddon.

Season 8
On August 12, 1958 Abaddon, possessing initiate Josie Sands, infiltrates the Men of Letters to steal The Box under the orders of an unknown party. She proves immune to an exorcism and kills all but Larry Ganem and Henry Winchester who escapes through time with the box.

Abaddon is able to follow Henry through time to the year 2013 where she is met by Sam and Dean. Dean manages to stab her with Ruby's Knife, but while it hurts her, Abaddon isn't killed and the three are forced to flee. Abaddon reads the memories of the motel clerk to learn what he knows about Sam and Dean and then kills him.

Abaddon travels to Astro Comics which used to be the Men of Letters headquarters and checks the memories of the clerk there. After learning about what she knows, Abaddon comments that she likes the clerk's "The Devil Made Me Do It" shirt and kills her, taking her outfit.

Using the clerk's memories, Abaddon tracks down Larry Ganem and apparently disguises herself as or possesses his wife and listens as Sam arrives and learns about the storehouse that the box opens and gets the coordinates to it. Abaddon kills Larry and knocks Sam out. Abaddon calls Dean and demands he trade Henry and the box for Sam's life which he agrees to.

At an old processing plant, they make the trade, but Abaddon betrays the deal, trapping Sam and Dean and mortally wounding Henry. However, they expected her betrayal and prepared for it: Henry shoots Abaddon in the head with a bullet engraved with a devil's trap though Abaddon doesn't realize what he has done at first. Abaddon is angered to discover that the box she was given is a fake and demands to know where the real one is. When no one tells her, Abaddon tries to read Henry's memories and discovers that she is unable to and unable to move far. Realizing what happened, Abaddon points out that they still haven't killed her, but Dean tells her that she will wish they did and decapitates Abaddon. Dean explains that with a Devil's Trap in her head, Abaddon is bound forever to her meatsuit so they will cut her into small pieces and bury them in cement, forever entombing Abaddon and defeating her.