Abaddon (Alaina Huffman) was a demon and a Knight of Hell who was responsible for the extinction of the Men of Letters in 1958, before disappearing for fifty-five years when she time travelled to 2013. Afterwards, Abaddon began crusade in the 21st century to take over Hell from Crowley and gain demonic power and control of the Earth, until she was killed by Dean Winchester with the First Blade.
Little is known about Abaddon's early past, but she was presumably originally a human who'd lived in the earliest times of humanity, then died and went to Hell, where her soul was mutilated and corrupted into a demon. Abaddon was personally selected by Lucifer to become a Knight of Hell, (As Time Goes By) and was trained with the other Knights by Cain. Cain, Abaddon and the other Knights of Hell subsequently spent centuries bringing and spreading chaos and destruction and committing horrific atrocities, until Cain left the Knights in the mid-19th century after falling in love with a human named Colette Mullen. (First Born)
When the Knights of Hell learned of Cain's union with Colette and how it had changed him and turned him from their evil ways, the Knights took Colette away in retribution. This in turn enraged Cain into hunting an slaughtering the Knights of Hell until only Abaddon was left. Abaddon possessed Colette, and when Cain tracked her down, Abaddon began killing Colette; Abaddon then fled Colette's still-alive body just as Cain stabbed her with the First Blade, resulting in Abaddon surviving and escaping and Cain unintentionally killing Colette instead. (First Born)
Destroying the Men of Letters
In 1958, when the demons that the priest Father Max Thompson was experimenting with a demon curing ritual to turn demons back into humans, Abaddon was sent to stop Thompson and his demon cure. On 5 August, Abaddon tortured and killed Father Thompson, but not before he told her about how he'd been working with Josie Sands, an initiate of the Men of Letters.
In the next week, Abaddon was possessing the Mother Superior of St. Bonaventure Convent in Milton, Illinois and was leading an underground demon operation there to extract captured humans' souls to make more demons. Henry Winchester and Josie Sands came to investigate (when one of the nuns left soulless after being included in the demons' soul extraction went insane and killed two others before committing suicide), and they and Sister Julia Wilkinson discovered the demons possessing the nuns; they managed to exorcise two of Abaddon's demons, but Henry was subdued in the scuffle against Abaddon due to Abaddon's immunity to exorcism rites as a Knight of Hell.
After learning Henry and Josie were Men of Letters, Abaddon intended to possess Henry so as to infiltrate the order and destroy it from within. But Josie offered herself up in Henry's place; Abaddon accepted, then possessed Josie Sands. Henry remained unaware that Josie was possessed or that Abaddon had now infiltrated the Men of Letters through her, and Abaddon posing as Josie convinced Henry that the soul-extracting operation was just a minor demon infestation that had been dealt with. Abaddon in Josie left with Henry the following morning, casually threatening Sister Wilkinson into keeping quiet about the latter's knowledge of the truth. (Mother's Little Helper)
On 12 August, on the night Josie and Henry were to be sworn into the Men of Letters, Abaddon aimed to steal the Men of Letters keyto the protected bunker that contained all of the order's collected and powerful knowledge. (As Time Goes By) Entering the Men of Letters' building through her secret possession of Josie, Abaddon single-handedly slaughtered all the elders of the order; except for Larry Ganem, who survived blinded. Abaddon then immediately afterwards went after Henry Winchester, who had the key to the Men of Letters' bunker, and followed him through a time spell from 1958 to a motel in 2013. (As Time Goes By)
After Henry and Abaddon arrived in 2013, while Henry fled with his future grandsons Sam and Dean in search of the other Men of Letters (whom it turned out were extinct in the 21st century due to the elders' massacre in 1958), Abaddon pursued them, killing anyone in her way. Abaddon eventually learned the bunker's coordinates and captured Sam when she caught up with him while he was talking with Larry Ganem (the last surviving Man of Letters), then killed Ganem. Abaddon called Dean and demanded that he exchange Henry and the key for Sam.
Abaddon met with Dean at a warehouse for the exchange and went back on her end of the trade, mortally wounding Henry and intending to kill Sam and Dean. But the Winchesters had counted on this and planned ahead - they gave her a fake key, and Henry was then able to, before dying, get close enough to Abaddon to fire a bullet engraved with a devil's trap into her skull; immobilising and binding her movement and powers. As nothing the Winchesters had could kill a Knight of Hell, Sam and Dean instead opted to keep Abaddon trapped, immobilised and powerless by decapitating her vessel and cutting it up into pieces, with the devil's trap bullet keeping Abaddon bound inside the vessel's immobile remains. (As Time Goes By)
Vendetta against the King
A matter of months after Abaddon was incapacitated by the Winchesters binding her to and decapitating her vessel, Sam and Dean awoke Abaddon by stitching her vessel back together, although they kept her arms cut off to stop her harming them, and the devil's trap bullet remained lodged in Abaddon's head and stopped her moving from a spot on her own. The brothers intended to perform Father Thompson's demon-curing ritual on Abaddon to complete the third and final of the Trials of God to seal Hell up, but Abaddon escaped by animating her detached hands to remove the binding devil's trap bullet from her skull, freeing her and allowing her to reattach her hands and escape.
However, Abaddon had learned from the Winchesters before escaping of how Crowley had become the King of Hell during Abaddon's 55-year absence between 1958 and 2013, and was shocked, enraged and disgusted that a dealing crossroads demon like Crowley was now controlling Hell. Shortly afterwards, when Crowley send out a distress call after being captured by the Winchesters for the third trial to close Hell, Abaddon heard and responded to the signal. She made it violently clear upon finding Crowley that she was unhappy with the current King and wanted a regime change, before Sam forced Abaddon to smoke out and flee by burning and destroying her vessel with holy fire. (Sacrifice)
Usurping control of Hell
A few days after Crowley was captured by the Winchesters, Abaddon (after having Josie Sands' body restored for her to repossess) quickly sought to take advantage of the power vacuum left among the demons by Crowley's absence, to usurp control as Queen of Hell. Abaddon also intended to, as Hell's new ruler, remove Crowley's dealing system and turn Hell into a conquering kingdom; with Abaddon having ambitions to, after taking over Hell, rule over humans and even angels as well. However, some of the demons Abaddon began recruiting to her cause were still unsure about turning to Abaddon and thought Crowley was still the rightful King; so Abaddon set about finding and capturing the Winchesters to lead her to Crowley, so she could kill him and become Queen of Hell.
To this end, Abaddon and the three demons she'd already recruited kidnapped two hunters - Irv Franklin and Tracy Bell - and held them in Eugene, Oregon to lure Sam and Dean into a trap. Sam and Dean took the bait and came to free Irv and Tracy, and Abaddon captured Dean and demanded he take her to Crowley under threat of becoming her new vessel. However, Abaddon was driven to flee upon learning an angel was present, when Gadreel manifested and killed her demon subordinates. (Devil May Care)
After this, Abaddon continued usurping power and control over Hell with Crowley incapacitated and imprisoned by the Winchesters; she doubled the number of Hell's souls by going back on all of Crowley's deals and contracts and collecting the souls early, and she intended to take Crowley's entire deal-making system for Hell down in her quest to reshape the demons into a force of conquest. (Heaven Can't Wait)
Power struggle and the First Blade
Though Abaddon continued usurping power among the demons with the aim of fully replacing Crowley and taking over as ruler of Hell, she still encountered some resistance, as many of the demons were either neutral or still loyal to Crowley. When Crowley was sighted with Dean and Castiel, and tracked by demons who reported this to Abaddon, Abaddon and her demons followed the trail and eventually located Crowley. However, Crowley recognised some of the demons were either uncertain or neutral, and turned his and Abaddon's power struggle for control of Hell into a voting campaign among the demons for which of the two would lead Hell; although Abaddon remained confident. (Road Trip)
Over the following weeks - months, Abaddon's side began to win in the power struggle, when Crowley's control was incapacitated while he was sunken in an addiction to human blood. According to Snooki, even Crowley's followers began to want to join Abaddon. Abaddon had Lola spy on the addicted Crowley as his concubine, and learned through Lola before Crowley killed her that Crowley and the Winchesters were searching for the First Blade to kill Abaddon with. Abaddon shortly after sent her demons after the Blade, but they failed to find it, and Crowley and the Winchesters ultimately managed to locate and seize the Blade first. (Blade Runners) Abaddon also had her soul-mining operations restarted, so as to make the extracted souls into an army of demons loyal to her; though the soul-mining operation at St. Bonaventure was put to an end when the last demon operative there was killed, it was apparently just one of many soul-mining ops Abaddon had. (Mother's Little Helper).
Negotiations and death
Later during Crowley and Abaddon's power struggle for Hell, Crowley's kingdom began losing to Abaddon's faction, with the majority of the demons including even Crowley's inner-circle turning to Abaddon. However, Abaddon still had to eliminate the First Blade and the Mark of Cain to ensure the Winchesters couldn't kill her and she could secure control of Hell. To this end, Abaddon contacted Crowley after the latter was betrayed over to Abaddon, and demanded that they call a temporary truce to stop the Winchesters killing them and destroy the First Blade. To force Crowley to agree, Abaddon used the same time travel spell that had transported her and Henry Winchester to the 21st century to abduct Crowley's human son Gavin from when he was alive in 1723 and bring him back to the present with her, as a bargaining chip against Crowley. This succeeded in forcing a reluctant Crowley to agree to join forces with Abaddon, when Crowley was unable to bear seeing Gavin suffer, due to the effects of an incomplete demon-curing ritual.
As part of Abaddon's plan to lure the Winchesters, through Crowley, to her location at the Humboldt Hotel in Cleveland, Ohio and into an ambush, Abaddon had Crowley call Sam and Dean and give them the First Blade's location and then Abaddon's to lure them and the Blade into the trap. However, once Crowley was of no more use, Abaddon double-crossed him and intended to kill him and Gavin with the Winchesters. Once Dean arrived with the First Blade and the Mark of Cain, he walked into Abaddon's trap and was captured, with Abaddon intending to kill him and destroy the First Blade to ensure her immortality and secure control as new Queen of Hell. However, the Mark of Cain gave Dean enough resistance against Abaddon's demonic telekinesis to break free and charge at her with the First Blade; successfully stabbing Abaddon with the Blade and destroying her once and for all. (King of the Damned).