The Emperor was able to muster the still-loyal Primarchs for reinforcements before Horus was able to defeat the forces of Terra. Here’s what happened.
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However despite these gains, the situation soon changed for the worse for Horus. He recieved news from his daemonic allies that the Space Wolves and Leman Russ had evaded the Alpha Legion, the Ultramarines under Roboute Guilliman had defeated the Word Bearers at Calth, and the Dark Angels under Lion El’Jonson were also Terra-bound. Horus was furious at the news. He had gambled everything on being able to defeat the forces of Terra before the Emperor could muster help from all the still-loyal Legions. With time against him, it now appeared that the gambit had failed and came up with an even riskier gambit. Lowering the teleportation-inhibiting Void Shields on his Battle Barge, Vengeful Spirit, Horus in effect invited the Emperor to come to him.
Despite objections by Rogal Dorn, the Emperor decided to willingly fall into Horus’ trap knowing all to well what awaited him. Leaving management of the Golden Throne to his trusted subordinate Malcador, the Emperor, Dorn, Sanguinius, as well as an elite force of Space Marine Terminators and Adeptus Custodes teleported directly into the Vengeful Spirit. However the teleportation process had malfunctioned and the loyalist forces were scattered about the daemonically corrupted ship.
Coming under attacks from all kinds of horrors aboard Horus’ flagship, including Daemons, monsters, and Chaos Space Marines, the loyalist forces fought their way across the Battle Barge in search of Warmaster Horus himself. The first to reach the traitorous Warmaster was Sanguinius himself. Refusing a final offer by Horus to join him in his campaign against the Emperor, the heroic Primarch found himself completely outmatched by the Warmaster, now warped by the powers of Chaos, and was struck down.
When the Emperor finally reached Horus, he witnessed to his horror the mauled corpse of Sanguinius at the traitorous Warmasters feet. Overwhelmed by sorrow, the Emperor was speechless as Horus mocked his once-beloved father, proclaiming himself the rightful ruler of humanity and claiming to have even bound the Chaos Gods to his will. The Emperor could only look on in pity, knowing all to well the trap that had ensnared his son. Stating that Horus had become a slave to Chaos, not the other way around, Horus became enraged and the great duel to decide the fate of humanity finally began.
Though a Psyker of unparalleled might, even the Emperor found himself greatly challenged by the might of Horus and the Chaos energies that swelled within him as the two engaged in a titanic battle of not only Power Sword and Lightning Claw but also psychic power. However even after all that he had done, the Emperor could not bring himself to use his full power and risk killing his beloved son. Using the immense power he had acquired during the Battle of Molech and imbued with the energies all four Chaos Gods, Horus crushed the Emperor’s windpipe, severed the tendons of his wrists, broke most of his ribs, plucked out his left eye, and set his hair alight. As if this weren’t enough, Horus lifted the Emperor high above his head and brought him down upon his knee, crushing the Emperor’s spine. Further punishing his father by ripping one of his arms from its sockets, Horus howled over the beaten body of the Emperor in triumph.
But suddenly, the beating stopped. A lone Custodian of the Adeptus Custodes had entered the room (while other sources state it was Ollanius Pius, an Imperial Army soldier). Without hesitation, the loyal warrior had charged towards Horus. Breaking out in laughter, the Warmaster blasted apart the warrior in a gaze of psychic energy. This proved to be the decisive moment in which the Emperor finally realized what his son had become. The Horus he had known was truly gone, destroyed by the maddening powers of Chaos. Realizing that he must kill Horus and he would never get another chance, the Emperor used the brief respite to lash out at Horus with his full power. Focusing his psychic bolt into a bolt of pure force, the lance of power blasted straight through the Warmasters heart.
Realizing their pawn had failed, the Gods of Chaos retreated from Horus’ body. As they did, sanity returned to the Warmaster. In that moment, Horus, realizing the atrocities he had committed, begged the Emperor to strike the finishing blow before the powers of Chaos would take hold over him once more. Focusing the last of his strength into another psychic bolt, the Emperor finally destroyed his repentant son.
Rogal Dorn and his Terminator escorts would reach the chamber shortly after, discovering the beaten body of the Emperor alongside the remains of Sanguinius and Horus. The Emperor only had enough strength to demand to be taken to the Golden Throne at once. Obeying his beloved master, Dorn and the surviving loyalist forces teleported off the Vengeful Spirit and back to the Imperial Palace. Meanwhile the death of their Warmaster had sent a psychic shock reeling across the traitorous armies on Terra. As the Great powers of Chaos retreated from Horus they took their Daemonic hordes with them. Confusion broke out among the traitor Primarchs and the rebel armies lost all cohesion. At the same time, the Blood Angels began suffering horrific visions of the death of Sanguinius. In what may be the first instance of the Black Rage, the Blood Angels broke ranks and began engaging the traitors in close combat with such savagery it shocked both loyalist and traitor Marine alike. Using this to their advantage, the loyalist forces led a decisive counterattack which finally expelled the traitorous legions from Terra in a bloody rout.
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