Battle of Gyros-Thravian
The Battle of Gyros-Thravian was a major battle towards the end of the Great Crusade that saw the forces of the Imperium pitted against the powerful Ork Warboss Gharkul Blackfang, known to be one of the most powerful Warbosses to have ever lived. Blackfang's army was attacked by three entire Space Marine Legions: the Imperial Fists, Luna Wolves, and Death Guard. Each was led by its Primarch: Rogal Dorn, Horus, and Mortarion. Despite this impressive force, the Space Marines were nonetheless on the verge of defeat at the hands of Blackfang.
It was then that the Emperor himself came to the aid of his sons. From his golden battle barge, the Bucephelus, the Emperor led one thousand Custodians directly into the heart of the Ork horde. Blackfang was slain by the Emperor personally atop his Gargant and the Custodians then laid waste to his horde, slaying over 100,000 Orks at a cost of only 3 of their own. After the battle, Blackfang's Waaagh! was shattered. It is said that the Emperor engraved the names of the 3 fallen Custodians into his armor.