Gharkul was a massive black-skinned Ork possessed of a great cunning. His stronghold was the planet of Gyros-Thravian; which would become the site of one of the Imperium's greatest victories of the Great Crusade.
The Legions of Horus, Rogal Dorn and Mortarion participated in the Battle of Gyros-Thravian. During the battle the vast horde of Gharkul came close to defeating all three of these legions, and they would have succeeded were it not for the intervention of the Emperor himself. From his golden battle barge, the Bucephelus, the Emperor led one thousand Custodians directly into the heart of the Ork horde.
At the height of the moment these Custodians would pass into legend. It is said over a hundred thousand of Gharkul's best warriors were slain in moments, and Gharkul himself was decapitated by the Emperor atop a ruined Gargant, ending the battle. Only three Custodians fell during the battle, and their names were later engraved onto the Emperor's armour.