Long War - The Horus Heresy: When the rebellion against the Emperor came, its leader was the primarch least suspected of any treachery: the great and noble Warmaster, Horus. Horus had always loved and respected the Emperor as a young man looks up to a strong father, and had faithfully remained at the Emperor's side throughout the Great Crusade.After Horus' stunning victories against the Orks during the Ullanor Crusade, the Emperor announced that He was returning to Terra, elevating Horus to the ultimate rank of Imperial Warmaster in his stead and giving him free rein to prosecute the Great Crusade as he saw fit, as he now possessed supreme command over all of his brother primarchs' Space Marine Legions.At the Triumph of Ullanor Emperor announced that Horus' XVI Legion, the Luna Wolves, could be renamed the Sons of Horus in honour of their mighty primarch. Yet the announcement bred jealousy and confusion in many of the primarchs. Did their father really intend to abandon them at this most critical time? Without explanation, the Emperor left for Terra in order to further His mysterious agenda, leaving Horus to deal with the crusade in His wake.Horus strove to carry out the Emperor's will to the best of his ability, and, either by chance or fate or sinister design, came to lead the Sons of Horus to the haunted swamps of Davin's moon, hoping to bring the Emperor's light to that benighted orb.There, beset on all sides by the servants of the Plague God Nurgle, Horus fell to a grievous blow from an eldritch sword, the Anathame. The tainted wound festered, plunging Horus into a fugue state from which he could not be awoken. The Sons of Horus were desolate at the loss of their founding father.Under the advice of Erebus, a well-respected Chaplain of the Word Bearers Legion, the most senior members of the XVI Legion took their primarch to the temple of one of the infamous Warrior Lodges of Davin, the Temple of the Serpent Lodge, for healing. Unbeknownst to the Space Marines, the Ruinous Powers restored Horus to his full strength, while cementing their grip upon the Warmaster's soul in the process by exploiting the small shard of jealousy and envy that Horus had always felt for his father the Emperor.Awed by the "miracle" worked by the Davinite priests, the Warmaster spread the new-found cult and the Chaotic practices of its Warrior Lodges throughout the Sons of Horus and beyond, infesting the Legions with heretical notions disguised as beneficence.Initially, even the mighty primarchs had little idea that they had fallen to the service of Chaos. When they rebelled, their initial good intentions and just causes began to fall away as their souls became saturated with the malign energies of the Warp. The Space Marine Legions that they led inevitably followed the fall of their commanders, blind to the faults of their genetic fathers and hungry for more conquest.The corrupting force of Chaos spread into the Imperial Army and the Mechanicum, and soon enough, the rot had seeped through to the core of the Emperor's new domain. As a new Age of Darkness fell across the galaxy with the onset of the Horus Heresy, hundreds of worlds burned in the name of the Dark Gods, and a terror unlike any seen before was unleashed across the galaxy during the seven dark standard years that the Heresy raged.Much of the truth of these times has been lost, obscured by the mists of time or embellished to the point where giants bestrode worlds with thunderous steps and the planets themselves cracked and split at their tread. The Traitor forces of Horus drove all before them, until those Astartes Legions still loyal to the Master of Mankind stood at bay upon the fortified walls of the Imperial Palace during the climactic Battle of Terra.The forces of darkness pressed in around the guttering flame of humanity, but desperate times called for desperate solutions. Sanguinius of the Blood Angels and Rogal Dorn of the Imperial Fists, together with their bravest warriors, would accompany the Emperor and take the fight to Horus upon his Battle-Barge the Vengeful Spirit, a mighty warship in orbit above Terra.The Emperor and His warriors teleported onto Horus' Battle Barge but found themselves separated and scattered throughout the corrupt vessel by means of the Warmaster's dark sorcery. The Emperor fought His way to the Warmaster but was too late to save Sanguinius, who Horus slew when the angelic primarch refused to turn to Chaos. Yet, some maintain that Sanguinius inflicted a wound, however small, upon his erstwhile brother.Horus and the Emperor clashed in a battle of both flesh and spirit. Horus was filled with the power of the combined Ruinous Powers and dealt the Emperor a mortal blow, but in the end, the Emperor's will was the greater, and Horus was struck down with the last ounce of the Emperor's strength. The Traitor was destroyed utterly, in body and soul and, with his death, the power of the Traitor Legions was broken.When Dorn and his warriors finally fought their way into the rebel Warmaster's sanctum, they found the Emperor's broken and ruined body, and it is said that their cries of woe were heard far below on Terra itself. Rogal Dorn, most determined and unbending of the primarchs, bore his master's body back to Terra and, under the direction of the crippled Emperor, bound Him within the strange psychic augmentation device known as the Golden Throne to sustain His existence for all eternity with constant human sacrifice and baroque machineries.The followers of the Ruinous Powers were defeated, but it was victory won at a terrible cost. The brotherhood of the primarchs was sundered, and the Emperor's vision for the Imperium and all of Mankind lay in ruins -- the last, best hope of its fulfillment lost forever.