Ezekyle Abaddon was the First Captain, leader of the First Company and official second-in-command of the Luna Wolves Legion, before it became the Black Legion. He looked upon Horus as a leader and a father, eventually revering him as a god and holding him above even the Emperor of Mankind.
Not only was he First Captain of his legion, whose exploits were known throughout the Imperium, he was also part of the choleric member of the Mournival, a group of four captains who acted as counselors to the Warmaster, and indeed had been in the group since its inception.
Extremely proud and quick to anger, Abaddon's martial record was unsurpassed by that of any other Luna Wolf. One of Abaddon's greatest glories was the final battle of the Ullanor Crusade, where only he and his Primarch Horus walked away from the speartip strike on the Ork Warlord's headquarters. He took his position extremely seriously, but did enjoy sharing his sense of humour with his Mournival brothers.
Imposing in almost every way, Abaddon possessed a particularly deep voice, was taller, stronger and bulkier than the average Space Marine, and wore his long black hair in a strikingly tall topknot when not using a helmet. He had similar facial features to those of Horus, particularly the widely spaced eyes and long, straight nose. He often wore a large black wolf-pelt over his power armour, to which he also attached a cloak upon occasion. Abaddon possessed both a suit of power armour and suit of Terminator Armour, the former being painted in the standard colours of his Legion, while the latter was primarily black in colour, which marked him out as a member of the 1st Company elite. Indeed, Abaddon's personal unit was the black-armoured Justaerin Terminator Squad, but he normally left command of it to Captain Falkus Kibre. Little Horus Aximand quipped that this was because Abaddon was actually too big to fit inside a Terminator suit. Towards the end of the Great Crusade, Abaddon would only be seen in his black Terminator garb.
Abaddon had a dislike of Aliens that was notable even for a Space Marine; his antipathy for them was so strong that he actually argued against the Warmaster's intention to parley with the Interex, a human civilisation that allowed an alien species to co-exist with them.
Abaddon eventually grew frustrated and angry at the Emperor's supposed 'abandonment' of the Space Marine Legions, feelings which swelled to the surface when Horus was mortally wounded on the moon Davin and laid near to death. Another Space Marine, Erebus of the Word Bearers Legion, who had secretly orchestrated Horus's near-death, proposed taking Horus to the shamanic Serpent Lodge for healing. Desperate for an answer to that wasn't coming from their own medical facilities, Abaddon readily agreed to this plan. And so had the begun the road to damnation,
The Horus Heresy
After Horus's recovery, he renamed his Legion to the Sons of Horus, the Legion grew ruthless and hateful. Abaddon grew ever more protective of his Primarch and his Legion's reputation, to the point where he saw remembrancer criticism and Imperial investigation into Legion activities as threats and insults that could not be borne. He therefore sanctioned the murder of the poet Ignace Karkasy and the blaming of Legion-caused civilian casualties upon Garviel Loken. This course of action was frustrated by the refusal of Tarik Torgaddon to accede to the plan, and the Mournival brotherhood was effectively broken as a result.
Abaddon's thinking would prove to be in tune with that of his commander, who would eventually order not only the death of Karkasy but also that of Loken. Seemingly destined to be in lockstep with the Warmaster, when Horus came to the warrior lodge of his Legion to sway them into following him onto what would prove to be the path of heresy, Abaddon was amongst the first to swear his unquestioning loyalty.
Abaddon was present at several of the major actions of the Horus Heresy. During the purging of the loyalist Legionaries at Isstvan III, Abaddon notably took to the field with Horus Aximand to attend the final meeting of what had once been the Mournival, at which they dueled with Garviel Loken and Tarik Torgaddon, respectively. Though slightly wounded in the combat, and Aximand fundamentally changed, Abaddon and Aximand emerged the victors, with Loken and Torgaddon presumed dead.
Seven years later, at the the Battle of Terra, the climax of the Horus Heresy, Horus and the Emperor had dueled on Horus's own flagship The Vengeful Spirit. Though the Emperor was mortally wounded, Horus was slain, definitively ending the Horus Heresy. The Sons of Horus recovered Horus's remains and retreated with the Traitor Legions to the region of space known as the Eye of Terror. Abaddon claimed Horus's lightning claw, the Talon of Horus, as his own along with the office of the Warmaster.
Abaddon eventually renamed the Sons of Horus as the Black Legion to expunge the name of Horus, who failed in his attempt to take over the Imperium. He eventually returned at the head of a diabolic horde which ravaged entire systems around the Eye of Terror before the Imperium could muster the strength to halt it. During this first `Black Crusade`, Abaddon made many bloody pacts with the infernal powers. In the crypts below the Tower of Silence on Uralan, Abaddon recovered the Daemon Sword Drach'nyen. With the howling daemon blade in his fist Abaddon became nigh unstoppable. Whole cities were burned in sacrifice to the ever-hungry daemons of Chaos, and entire armies were torn apart by gibbering warp entities. Abaddon's power swelled to inhuman proportions as the gods of Chaos rewarded him lavishly and he undertook acts of fiendish bravery which horrified those who stood against him. During the Gothic War Abaddon almost brought an entire sector to its knees. His fleets were augmented with a newly constructed flagship, known for good reason as the Planet Killer. Alongside this he somehow activated and gained control of the Blackstone Fortresses, mysterious constructions allegedly рre-dating the Imperium itself that combined to generate prodigious destructive firepower. Abaddon attacked while the sector was cut off from reinforcements by warp storms and caused huge damage to the Imperial battlefleet, destroyed a number of planets and devastated many more. Only the intervention of the Eldar enabled Imperial forces to stop the Chaos fleet. In the most recent assault, Abaddon managed to gain a foothold in the Cadian Gate, although he was unable to consolidate his gains and they have largely been snatched from his grasp by a vicious counterattack from the Imperial Navy. Although his ultimate victory has thus far been prevented, the High Lords of Terra live in fear of the day that Abaddon unites all of the Traitor Legions into an unstoppable horde and returns to play out the last acts of treachery begun by Horus ten thousand years ago. Should Abaddon's armies triumph, the floodgates wull be open and a never ending tide of Chaos will pour out of the Eye of Terror to despoil the Imperium, and perhaps once again strike at the most prized of worlds-Terra.