The Battle of Gyros-Thravian was a campaign of the Great Crusade fought in the last century of the 30th Millennium against the extremely powerful Ork Warlord Gharkul Blackfang, one of the most powerful Ork leaders ever encountered to that time. Three full Space Marine Legions, including the Luna Wolves, Death Guard and Imperial Fists, were deployed to face the Greenskin menace. Despite the strength arrayed against the Orks, it was the Imperial forces who were soon on the verge of defeat.
It was then that the Emperor Himself, aboard His orbiting flagship Bucephelus, came to the aid of His Primarch sons and their Astartes. He personally led a force composed of 1,000 Legio Custodes into the heart of the mighty Ork horde. Blackfang was confronted by the Emperor and killed atop his Gargant while the Custodians proceeded to lay waste to the rest of the Greenskin horde.
The Custodians accounted for the slaughter of the Orks, slaying over 100,000 of the savage xenos, with the loss of only three Custodians. Following their momentous victory, the Emperor commemorated their sacrifice by engraving the names of the three fallen Custodians into His personal Power Armour.
- The Horus Heresy: Collected Visions (Art Book), "The Custodian Guard," pg. 143