- "The ancient knights’ quest for the grail, the alchemist’s search for the Stone of the Philosophers, all were part of the Great Work and are therefore endless. Success only opens up new avenues of brilliant possibility. Such a task is eternal and its joys without bounds; for the whole universe, and all its wonders… what is it but the infinite playground of the Crowned and Conquering Child, of the insatiable, the innocent, the ever-rejoicing heirs of the galaxy and eternity, whose name is Mankind?"
- —The Book of Magnus
The Book of Magnus, also referred to as the Book of the Thousand Sons, is a massive tome containing the collected knowledge of sorcery as dictated by Magnus the Red, the Primarch of the Thousand Sons Legion, during the Great Crusade in the late 30th and early 31st Millennia. It is one of the most complete treatises on psykers, witchcraft and sorcery in the galaxy, a compilation of all the knowledge and experiments gathered by Magnus during the conquests the Thousand Sons made for the Imperium during the Great Crusade. This esoteric tome was copied down from the Primarch's own words by the Remembrancer Mahavastu Kallimakus, a meticulous recorder of data and a fastidious observer of details. He had recorded much about the earliest days of the Great Crusade as he had been one of the first Remembrancers to be chosen to accompany the Thousand Sons. His reputation had preceded him, and he was immediately assigned to Magnus the Red as his personal scribe, recording the Primarch's every thought and deed. He stood at his master's side since the restored XV Legion had departed Prospero to be a part of the Emperor's Crusade to reunite the disparate human worlds into a single Imperium of Man.
The Book of Magnus was a heavy book bound in faded leather and stitched in gold. An iron pendant, worked in the form of a snarling wolf's head against a crescent moon, lay along the valley at the meeting points of its pages. It contained the distilled wisdom of all that Mahavastu Kallimakus had written in his long years of unthinking service to the Thousand Sons. To a heavy lectern of cold iron within the Sanctum of Magnus on Prospero was chained the grimoire of all of Magnus' achievements, culled from the unexpurgated texts penned by his scribe. The Book of Magnus always remained inviolate, protected with arcane spells and powerful psychic wards, its pages impervious to fire. To look upon it was an honour, but to hold it and be expected to read from its pages was the culmination of a lifelong dream for only a select few.
The massive tome currently resides in the Daemon Prince Magnus' library on the Planet of the Sorcerers within the Eye of Terror, carefully kept out of reach. It it is rumored that another copy is held within the Black Library of Chaos maintained somewhere within the Webway by the Eldar. How the Eldar managed to obtain a copy of this esoteric tome is unknown.
Before the Horus Heresy, Ahzek Ahriman, the Chief Librarian of the Thousand Sons Legion and the second most powerful sorcerer within the XV Legion after Magnus, was entrusted with the guardianship of the Book. However, after the start of the Heresy, once Magnus ascended to daemonhood, and the dreaded mutational "flesh-change" started to run wild amongst the Thousand Sons once again, Ahriman delved deep into the pages of the tome that were filled with forbidden lore and knowledge from ancient, forgotten days. He believed the Book of Magnus held the key to his Legion's salvation. Using the labyrinthine collections of formulae, incantations and rites, Ahriman devised what he believed would be the beginnings of a mighty arcane spell to undo all that had befallen his Battle-Brothers, a spell he called the Rubric of Ahriman. Since that fateful day when Ahriman's spell was cast and forever shaped the destiny of the Thousand Sons, Magnus has kept the secrets hidden within the Book to himself, unwilling to deal with another cataclysm falling upon his Legion as a result of a misuse of his greatest work.
- Realm of Chaos: The Lost and The Damned, pg. 267
- White Dwarf 230 (US), "Bitter and Twisted: Ahriman"
- White Dwarf 227 (US), "Chapter Approved - Ahriman: Chaos Sorcerer, pp. 73-80
- White Dwarf 202 (US), "The Sorcerer: Tzeentch, Ahriman", pp. 9-14
- A Thousand Sons (Novel) by Graham McNeill, pp. 79, 193-194, 206, 211, 214, 259, 261-262, 264