Abaddon, also known as Abaddon the Despoiler, once named Ezekyle Abaddon, is the Warmaster of Chaos, a Chaos Lord and the greatest Champion of Chaos Undivided in the galaxy.
Abaddon is the master of the Black Legion of Chaos Space Marines and is rumoured to be the clone-progeny of the Warmaster Horus, the greatest Traitor in Imperial history, and at one time his most favoured son as the first captain of the Space Marines of the Sons of Horus Legion.
Abaddon is now infamous for leading Black Crusades, the terrible military campaigns during which the normally fractious forces of Chaos unite under his leadership and launch a massive attack against the Imperium from within the Eye of Terror.
The most recent of these attacks, the 13th Black Crusade in 999.M41, led to the fall of the vital Imperial Fortress World of Cadia, the birth of the Great Rift that divided the galaxy in half and the start of the Noctis Aeterna and the Era Indomitus.