Abbadon the Despoiler is the Warmaster of the Black Legion, and leader of the 13th Black Crusade.
Official Canon Edit
Abbadon was originally the First Captain of the Sons of Horus, having fought many battles in the Horus Heresy. When Horus was defeated at the Siege of Terra, though, the Traitor forces retreated into the Eye of Terror. With the legion now broken and without a leader, many broke off into other war bands until Abbadon reformed the Sons of Horus into the Black Legion, and cemented his leadership by launching the first Black Crusade.
There have been thirteen Black Crusades in total; originally they were all attempts to seize Cadia, the only secure way out of the Eye of Terror, but this has since been retconned. Each Crusade was a separate operation with its own goals; in the twelfth Black Crusade, Abbadon obtained the Blackstone Fortresses, which he used I the thirteenth Black Crusade to smash Cadia's pylon network and allow the Eye of Terror to expand. His ult Mate goal is to eventually reach Terra and finish what his father started by killing the Emperor.