The Ullanor Crusade was a vast assault upon the powerful empire of Ork Overlord Urlakk Urg during the Great Crusade. It is remembered as Horus's greatest victory while he was still loyal to the Emperor and the act that earned him the title of Warmaster.
Assault on Ullanor
While the Ultramarines and White Scars Space Marine Legions, supported by newly-raised Imperial Army regiments, were tasked to retake the outlying planets of the Ullanor system, the Luna Wolves under Horus had their fleet drove straight for the central world. The force was massive: one hundred thousand Space Marines, eight million Imperial Army troops, and hundreds of spacecraft.
Employing his favoured tactic of a direct strike at the enemy command structure, Horus unleashed his Legion. Drop pods crashed to the ground all around Urg's fortress-palace. Heavy shuttles deployed Land Raiders and Predators and armoured Space Marines advanced on the defences. Then, as hundreds of orks rushed to join the battle on the perimeter walls, Horus and a Terminator detachment teleported directly to the foot of the great central tower. As the Luna Wolves blasted away the guards, mobs from the walls raced back to protect Urg. Horus took ten of his Terminators up the tower, leaving the rest to hold back the orks.
At the pinnacle of the tower, they found Urg in a grand chamber accompanied by forty of the biggest orks in his empire. Horus charged straight toward the warlord, slicing through muscled green bodies with his lightning claws. The Terminators with him similarly crashed into combat. Slowly, they hacked a path through the mob until Horus faced Urg himself. The enormous Overlord was simply no match for the Primarch's skill and unnatural power. Horus crippled his enemy, hefted Urg's broken body out onto the roof, and threw him screaming from the battlements to fall far below, amongst the horde of orks still assaulting the lower levels.
The sudden demise of their mighty leader sent a panic through the Greenskin forces, which fell back from the Terminators into the advancing Luna Wolves and were slaughtered. Back in the Overlord's chamber. Horus found his First Captain Ezekyle Abaddon standing alone and gore-drenched amongst the crushed and broken bodies of the Justaerin and the orks.
As word of his death spread, the Overlord's empire fragmented. The Imperial forces retook control of the quadrant within a year. Naturally, the Luna Wolves claimed to have liberated substantially more worlds than their allies. Not long afterwards, the Emperor of Mankind declared that a great Triumph would be held to recognize the glory of the Ullanor Crusade. At this event, the Emperor named Horus Warmaster and that his legion had earned the right to be called the Sons of Horus. He also announced his retirement from the Great Crusade, expecting that the Imperial forces would obey Horus's commands from then on as if they were his own.