Horus was the first Primarch to be rediscovered by the Emperor during the early Great Crusade. Because of this early discovery, Horus grew to be the most powerful among the Primarchs as he had grown up from a child to an adult at the side of the Emperor. For a long time he was the only Primarch to have been discovered. Friendship between the Emperor and Horus grew rapidly and the Emperor eventually trusted him enough to give him command of the entire force of the Imperium. The Emperor had saved Horus's life at the siege of Reillis as they fought back to back. At another battle, Horus repaid this debt when he hacked the arm off a frenzied Ork as it tried to choke the life out of the Emperor.
The Great Crusade
As the Emperor and the Great Crusade marched on, Horus proved himself to be a tactical genius. He knew precisely which force to send and where to send it, showing no mercy to those that opposed the Emperor but sparing the innocent from unnecessary bloodshed. After the Ullanor Crusade, in which an Ork empire was defeated, the Emperor, considering the crusade to be Horus' greatest victory yet, saw fit to partially transfer control of the Great Crusade to Horus, raising him to the rank of Warmaster, the highest official beneath the Emperor himself, and granting him command over any and all Imperial forces, as well as other rights and honours. In these, the last years of the Great Crusade, Horus would encourage the other Primarchs to compete against each other in order to discover the strongest and most able of his companions and to improve their fighting abilities, as well as lead his own personal forces into several notable campaigns. It was after one of these campaigns - the first contact with the Interex - that Horus chose to exercise his right, granted by the Emperor, to rename his personal legion: The Luna Wolves were renamed the Sons of Horus.
The BetrayalDuring a mission on the Feral World Davin, Horus was wounded in battle, but the natives of the world helped to heal him.
Many modern scholars believe that the group of healers were actually Chaos Cultists who somehow managed to take advantage of the illness in the Primarch. After this healing, changes in Horus began to occur. It seems from then on, Horus planned and plotted to overthrow the Emperor in what was to become known as the Horus Heresy. During his illness, he was inducted into a "warrior lodge." Although the ceremony was intended to make him a "brother" rather than an outsider, the lodge was supposedly rife with Chaos taint, and it accelerated his change. He introduced the taint to the Legions under his direct command, and it spread until it had infected a full half of the Space Marine Legions of the Imperium. Horus also introduced the concept of deposing the Emperor and installing himself, a stronger leader willing to take the fight to the enemies of Humanity in his place. Horus moved, along with several other secretly traitor legions, to Isstvan III, seemingly in order to suppress a rebellion and reinstate Imperial control. Once he arrived however, he sent down specially selected troops from all four of the legions with him, sending down all those he knew would never join him in open rebellion. Once they were on the planet he proceeded to virus bomb the entire world and killed billions of inhabitants in seconds, along with hundreds of space marines, and a ground war eliminated the remainder. The psychic shock wave from this event was said to be louder than the Astronomican. Horus then redeployed his forces to Isstvan V where he was met by seven Space Marine legions sent to bring him to Terra to face an inquest into his actions on Isstvan III. Four of these seven legions proceeded to rebel against the Emperor. It now became obvious that Horus had massive power as he hunted down the three legions that had stayed loyal. Only a few Space Marines managed to escape his forces and make it back to Terra, and among those killed was Ferrus Manus, Primarch of the Iron Hands. Horus then set a course to fight his way through the Imperium in an attempt to reach Terra and, ultimately, to kill the Emperor and place himself on the throne. After much fighting, death and betrayal, Horus realized he would have to hurry his attacks in order to secure Terra in time for him to set up the defenses sufficiently to prevent the arrival of two loyal Space Marine legions which could potentially turn the balance back in the favor of the Emperor. To this end, Horus ordered the void shields of his flagship dropped to entice the Emperor aboard for one final conflict. Somehow, the loyalist forces arrived scattered throughout the ship, and Sanguinius, Primarch of the Blood Angels, appeared before Horus, who offered him riches and power if he denounced his allegiance to the Emperor. Sanguinius refused and attacked. Horus killed him and was found standing over his broken body when the Emperor entered the chamber. The Emperor and Horus fought with a power that would have eviscerated any mortal man dozens of times over with each blow. The Emperor held back for much of the battle, remembering Horus as his beloved son and not wishing to believe that he had turned so utterly to Chaos. This allowed Horus to inflict crippling mortal wounds on the Emperor, since nothing short of the Emperor's full power would be sufficient to defeat him. At the critical point in the battle, a lone Adeptus Custode guard entered the room. Horus flayed him alive with but a look and in that instant the Emperor realized how far his favored son had fallen. The sacrifice of the Custodian bought the Emperor time to deliver a very powerful strike to Horus.
After this, Horus' sanity returned to him. Understanding all atrocities he had done, Horus asked his father to kill him, as Forces of Chaos would try to corrupt him again, and he wouldn't be able to resist. Then, the Emperor, with iron resolve, he gathered his full strength and delivered a massive psychic blow that killed Horus instantly and obliterated his very soul.
After the battle, the warship was won back by the forces of Chaos led by Abaddon the Despoiler, Captain of the 1st Company. Abaddon found Horus's body and ordered the retreat into the Eye of Terror. The body of Horus is said to have been put on display in a temple, the Sons of Horus revering the Warmaster even in death. At the end of a series of inter-traitor wars, the fortress of the Sons was destroyed, and the body of Horus stolen by the Emperor's Children. To the disgust of the Sons, the body was used to create at least one clone of Horus. The Sons, renaming themselves the Black Legion subsequently attacked the cloning facility and destroyed Horus's body and clone. Horus' famous Talon of Horus was taken by Abaddon and is the only part of the Warmaster remaining.