Horus - Endgame: The Siege of Terra following the initial assault on the Imperial Palace lasted for 55 solar days. Both sides knew that the defeat of the Imperium of Man was near after the defence of the Eternity Gate. Sensing this, and knowing that he must complete the siege before the arrival of Loyalist reinforcements from the other Space Marine Legions that were already on their way, Horus prepared to teleport to the surface from his flagship, the Vengeful Spirit, to lead his forces in person. Before this could happen, the Word Bearers' First Chaplain Erebus broke the news to Horus: their daemonic allies in the Warp had informed them that the Dark Angels and Space Wolves Legions were nearing Terra; and the Ultramarines were only a short distance behind.At that moment, Horus despaired; his gamble had failed, weeks of further conflict would be needed to break the defenders and the Emperor's reinforcements would arrive in mere hours. What happened next is disputed in the Imperial historiography of the Heresy; some believe Horus disabled his Void Shields as he experienced one last moment of regret for his betrayal of his father and his turn to Chaos, while others believe it was a personal challenge to the Emperor. Nevertheless, Horus lowered the Void Shields of his flagship, the enormous Battle Barge Vengeful Spirit. The lowering of the vessel's shields was detected by the Loyalist vessels in orbit and the information was relayed to the Imperial Palace.The Emperor of Mankind rose to the challenge, leading members of His elite personal guard, the Legio Custodes, the Primarchs Sanguinius and Rogal Dorn, and several companies of Imperial Fists and Blood Angels Veteran Space Marines in the assault and teleported aboard the Vengeful Spirit. Horus used his Chaotic powers to scatter the Emperor's force throughout the massive warship when they teleported up through the Warp. Each fought a series of battles against the elite Forces of Chaos aboard the corrupted starship, attempting to link up with their comrades and confront Horus.It was Sanguinius who reached his brother Horus first. The Warmaster attempted to turn the Blood Angels' Primarch, his oldest and closest friend among the other primarchs, to Chaos one last time. When Sanguinius refused to be corrupted, Horus attacked. Wounded from his many battles on Terra and the terrible battle with the daemon Ka'bandah, Sanguinius proved to be no match for Horus, now at the peak of his daemonic power after his long alliance with the Ruinous Powers. Horus strangled the Angel of Baal with ease. An alternate version of this event sometimes recorded in the Imperial records has Sanguinius cutting a small hole in Horus' Terminator Armour before he died, as this hole aided in the Emperor's final defeat of Horus.When the Emperor finally entered the throne room of the Vengeful Spirit, he saw the winged corpse of the angelic Sanguinius lying at Horus' feet. Horus called the Emperor foolish for refusing the power that the Chaos Gods offered to Men, and timid for not taming them to His will if He was truly the Master of Mankind as He claimed. Horus proclaimed that if the Emperor would kneel before him, then he would spare his life. But the Emperor, tens of millennia older than his misguided and once beloved son, knew well the ancient trap that had snared Horus. The Emperor told the corrupted Primarch that he was the deluded slave of Chaos, not its master, for no mortal could ever truly claim to be more than simply a pawn of the Ruinous Powers. Snarling, Horus hurled bolts of daemonic lightning at the Emperor, but the Emperor nullified them with His own immense psychic abilities. The die was cast. Each god-like being knew that the fate of humanity now hung in the balance.The Emperor and Horus engaged one another in the throne room of the massive Battleship, a battle that was both physical and psychic in nature. Though the Emperor's psychic gifts and martial skills were unequaled, He found Himself unwilling to summon His full strength against His beloved son. The Emperor suffered grievous wounds at Horus' hands, and after a score of thrusts, parries and counter-thrusts between the Emperor's Runesword and his own Lightning Claw, Horus sliced open the Emperor's chest armour, then opened his jugular and severed the tendons in his right wrist, disarming the Emperor. A psychic blast seared the flesh from the Emperor's face, destroying one of the Master of Mankind's eyes. After tearing the Emperor's right arm from its socket, Horus raised his father's broken body high over his head, and broke his back over his knee.At that moment, a lone Loyalist warrior entered the bridge, just as the Emperor fell. Horus, gloating in victory, showed the Imperial warrior the Emperor's broken form and laughed, taunting the Loyalist's uselessness and defeat. The valiant Imperial warrior, far from being daunted, roared in defiance and interposed himself between the Emperor and the Warmaster, heroically holding the line against the Chaos-corrupted Primarch. He was flayed alive for his defiance by a glancing psychic blast from Horus.The exact identity of the brave warrior is a source of much debate amongst Imperial historians, for today only the Emperor knows the truth. The Adeptus Ministorum tell the story that it was a lone Imperial Army trooper named Ollanius Pius who held the line, going as far as to canonise the man as the Guardian Saint of the Imperial Guard in the 32nd Millennium, yet many doubt that mere humans would have accompanied the Emperor and His Primarchs to confront Horus. The Imperial Fists maintain it was one of their Terminators who challenged Horus, however some see this as a desperate attempt to lessen their collective guilt stemming from the fact that Rogal Dorn never could participate in the fighting. Most savants now agree that it was probably one of the Adeptus Custodes who interceded, yet the other stories continue to be told and retold over the whole of the Imperium of Man.The casual brutality of the Warmaster's act galvanised the Emperor as He realised what awaited Mankind under the rule of Horus and the Chaos Gods. Realising at last that His favoured son was truly lost to the corruption of Chaos, the Emperor finally gathered His full and awesome psychic power in the Immaterium and unleashed a lance of pure Warp energy that pierced the gloating Horus' psychic defences and ripped his body apart. In some versions of the tale, this blast was only able to pierce Horus' body through the hole that had been made by Sanguinius before his death. Just before Horus died, he looked his father in the eye, shedding a single tear, begging his father to forgive him for his betrayal. The Emperor saw regret in his fallen son's eyes. The Emperor also knew that the Ruinous Powers could attempt to possess Horus again, and that He would not be there to stop his son again if they did. Driving all of the near-infinite reserves of compassion from His mind for the sake of the humanity He had served and loved all the years of His long life, the Emperor destroyed Horus utterly, his essence burned from existence in both the physical world and the Immaterium so that the Ruinous Powers could not resurrect Horus as a Daemon Prince through their claim on his soul.The destruction of Horus' soul sent a psychic shockwave surging across the Solar System, casting the daemons of Chaos back into the Warp, and spreading mass panic among the Traitor Legions and other Traitor forces on the surface of Terra in seconds as the Chaos Gods found their powers disrupted temporarily by the death of their favoured mortal vessel. It became clear to the Forces of Chaos that their leader had been defeated. A terrible, berserk fury later known as the Black Rage had encompassed the Blood Angels at the moment of their Primarch's death, and they were surging forth to scatter the attackers. Retreat turned to rout, and rout soon turned to bloodbath; thousands upon thousands of Chaos Space Marines and Chaos Titans fell attempting to flee. The ground before the Sanctum Imperialis ran red with the blood of Traitors and Heretics.