6th Black Crusade

During the 6th Black Crusade launched in 901.M36, Abaddon the Despoiler visited his wrath upon the Chaos Lord Drecarth the Sightless and his treacherous warband known as the Sons of the Eye. The Sons of the Eye were a splinter warband of the Black Legion, founded and commanded by Drecarth the Sightless, the Blind Butcher of Irridous VII. Drecarth was a former Battle-Brother of the Sons of Horus who had fought alongside Abaddon during the Great Crusade and the Horus Heresy that followed. Always jealous of the Primarch Horus' favour for the Despoiler, the Sons of the Eye considered Abaddon a usurper. Abaddon aided the Sons of the Eye in an assault on the Forge World of Arkreach, fighting side-by-side with them until the Adeptus Mechanicus defenders were finally crushed. The triumphant Abaddon then slowly killed Drecarth, impaling him with the Talon of Horus, even as the Despoiler made him watch every member of the Sons of the Eye bow before their new master.

History
When Abaddon ascended to command of the Sons of Horus, not every warrior of the XVI Legion swore allegiance to him. Many of the Traitors clung to their worship of Horus as a god, believing that he would one day return to lead them and punish those who had forsaken their oaths. Others considered the Horus Heresy to be the end of their subservience to gods and masters; the Emperor and their Primarch were the last overlords they would ever bow down to and they saw no reason to make an exception for Abaddon. Most of these Renegades were gradually lost to the Warp, disappearing into the Eye and vanishing from record, though some prospered and would return to be a thorn in the side of Abaddon.

One of these splinter warbands was the Sons of the Eye, led by the Chaos Lord Drecarth the Sightless. Drecarth the Sightless formed the Sons of the Eye out of those Sons of Horus who refused Abaddon's oath. A former Battle-Brother of Abaddon's, Drecarth had been one of Horus' captains, escaping in the chaos after Maeleum fell. Drecarth and his warband fled deeper into the Eye, only to return time and again to launch raids against multiple Daemon Worlds as well as other Renegade warbands. They initiated the Battle of Keening Deep in 892.M32, where the Sons of the Eye defeated the Chaos Lord Voslok and his World Eaters warband, claiming their fortress and hurling their foes' skulls into the world's soulfires. Drecarth personally ripped Voslok's Daemon Axe from his grip during the battle, turning the treacherous weapon upon its former master.

Abaddon had heard whispers of Drecarth's escape and treachery from his cabal of Chaos Sorcerers, who also claimed that an old ally would one day rise to subvert the Black Legion, twisting its loyalty with the memory of its dead Primarch Horus. So, under the guise of a truce, Abaddon made a pact with the Sons of the Eye and allied with them during the 6th Black Crusade in 901.M36. Abaddon wanted to make an example out of the Sons of the Eye, one that would serve as a dire warning to any that would consider challenging his power, but he needed to set the stage for his vengeance so that none would ever doubt his resolve.

During the 6th Black Crusade, Abaddon besieged the Imperial Forge World of Arkreach, offering Drecarth and his Sons of the Eye an equal share of the plunder. For months, the two forces of Chaos Space Marines fought side-by-side against the defences of the Adeptus Mechanicus, bombarding their great forge cities from space. Finally, the Traitors stood triumphant in the smouldering ruins of the great manufactoria, dead littering the ground. As Drecarth extended his hand in greeting, Abaddon grasped it with his own, only to thrust the claws of the Talon of Horus into his fellow Chaos Space Marine's gut. Drecarth lived long enough to see the Sons of the Eye bow to Abaddon and be reabsorbed into the Black Legion before the Warmaster of Chaos tore out his skull and spine. Thus did Abaddon deliver a dire warning to any who dared challenge his power.