Little is known of Magnus's early life, but it is known that he was discovered on the hive world of Prospero, where he led a small sorcery cult. It is said that, some time during this period, Magnus cut out his eye as a sacrifice to gain dark knowledge. The Emperor dissolved the cult and made him the primarch of the Thousand Sons instead, but he reintroduced sorcery into his ranks quite often. He always used his magic to benefit the Imperium, which was the only thing that kept them from being dissolved, but the Legion was constantly in trouble.
While meditating on Prospero, Magnus foresaw Horus being corrupted by Chaos and the future events of the Horus Heresy, the betrayal of half the Space Marine Legions, and the sundering of the Imperium. The only fate the vision did not mention was Magnus's own. Burdened with the information by this precognitive vision, Magnus used the power of his Legion's greatest sorcerers to convey the news of the impending civil war to the Emperor himself via sorcery, rather than the unreliable means of Astrotelepathy.
Upon receiving the message, the Emperor was furious at Magnus's oath-breaking, particularly because the strong psychic message broke the seal on his webway project, an attempt to break into and use the Eldar webway that would render warp travel obsolete. The Emperor saw Magnus as the traitor, not his beloved Horus. Leman Russ, Primarch of the Space Wolves Legion, who had always been averse to witchcraft, was ordered swiftly to bring Magnus to Terra to account for his actions. During his voyage to Prospero, Leman Russ was ordered by Horus to destroy Magnus's Legion rather than simply bring them to account. Accompanying the Space Wolves were with a full contingent of Adeptus Custodes, and the elite anti-psyker (pariah gene-bearing) units, the Sisters of Silence.
Ironically the Burning of Prospero took the Thousand Sons by total surprise. Landing on Prospero, the Space Wolves destroyed the capital city and with it the Thousand Sons' hard-won knowledge sending Magnus into a deep melancholy. In the close-quarter melees that followed, Magnus and Russ met in single combat. It was a battle of titanic proportion in which Magnus used his sorcerous powers on Russ. Russ was by far the better fighter but with Magnus using powers from the warp, the battle was evened up. At the climax of the battle, Magnus shattered Russ's breastplate, puncturing one of the Primarch's hearts. Russ's might was too great though and he countered with a salvo of thunderous combo's and a final earth shattering head butt that bloodied Magnus's single eye and fractured his skull. With Magnus dazed, Russ seized Magnus's broken body and lifted him into the air, before bringing him down over a bent knee, breaking Magnus's back. Horus had predicted that the attack would convert the Legion to Chaos, and so it did.
To save himself, his Legion, his world, and all the knowledge he had accumulated, Magnus pledged himself to the Chaos God Tzeentch. His new patron's response was immediate. The City of Light was transported into the Eye of Terror. While Prospero was destroyed that day, Magnus and his Legion survived. By the time the the Thousand Sons were seen next, they were allied to Horus, and Magnus had become the most powerful of all Tzeentch's daemonic servants.
After the Emperor defeated Horus, the Thousand Sons claimed a planet for themselves within the Eye of Terror. Now, Magnus's ever-changing form resides on the Planet of Sorcerers. Here, Magnus stands atop his tallest of towers, The Tower Of The Cyclops, and his vast sorcerous eye (possibly "borrowed" from Tolkien's Sauron) surveys the entire planet. The world and its buildings are places where dimensions exist beyond the norm.
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