Before the events of the Horus Heresy, Ahriman had shared his Primarch's obsession with arcane mysteries and had come to be keeper of the now mythical 'Book of Magnus', a tome of incalculable sorcerous power.

It may have been knowledge gleaned from that very tome with which Ahriman conjured his now infamous master spell. The result of the so called 'Rubric of Ahriman', while imperfect, is said to have satisfied Ahriman in its consequence. A veteran Thousand Son from before the coming of Magnus the Red, Ahriman's revulsion at the corruption of the Legion was so great that even the terrible price of reversing it was not too high.

Magnus was not of like mind, however. So great was his wrath when the cabal was discovered that the Primarch threatened to obliterate them utterly, but the very patron who had worked the mutations upon them in the first place was said to have intervened. Who can say what the most enigmatic and capricious of entities intended? In any event, the Daemon Prince stayed his hand, instead banishing Ahriman from the Planet of Sorcerers and condemning him to wander the Eye of Terror and beyond in a hopeless quest to understand the Chaos god Tzeentch.