Magnus had the intention of destroying the errant sorcerer, for despite the Chief Librarian's considerable powers he could not match the Primarch. The fallen Primarch's fury knew no bounds. As he raised his fist to deliver the single killing blow, a distant and sibilant voice reverberated with the knowledge of thousands and the machinations of something altogether beyond his understanding; "Magnus ... You would destroy my pawns too readily."
Realising the Architect of Fate, Tzeentch itself, had orchestrated the entire event, Magnus spared Ahriman, realising there were yet still things beyond even his elevated understanding. The Thousand Sons' Chief Librarian was banished on an eternal quest for perfect understanding -- leading him to take up such dangerous and enticing quests as the hunt for the Eldar Black Library.
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Lalalalai.... I'll cut you up!