Mahavastu Kallimakus was one of the first Remembrancers to be chosen by the Thousand Sons to accompany their Expeditionary Fleet during the Great Crusade in the 30th Millennium and early 31st Millennium. An astute scholar, he quickly rose in station, becoming the personal scribe of the Primarch Magnus the Red. Recording the Crimson King's every thought and deed, this knowledge, along with Magnus' collection of lore and lost knowledge of those civilizations brought into Imperial Compliance, were compiled into a single arcane tome -- the Book of Magnus. Realising that much his writings within the Book of Magnus were, in actuality, a series of mystic rites, Kallimakus fled him homeworld of Prospero before censure was brought against due to Magnus' breaking the Edicts of Nikaea.
Following the Horus Heresy, Kallimakus' writings eventually found their way to the world of Apollonia, where they were guarded by a fanatical sect of warrior-priests. Sometime in the early 32nd Millennium, the former Equerry of Magnus, Ahzek Ahriman, traveled to Apollonia. Kallimakus' tomes were held in a grand citadel known as the Athenaeum, and contained records of the Thousand Sons from the time of the Great Crusade. After a solar month-long siege by Ahriman's forces, the defenders lay broken before the Arch-Sorcerer's feet. Ahriman plundered the writings from their vault before burning the library to ashes, so that he alone would know the secrets contained within their pages.
Mahavastu Kallimakus hailed from Prospero, from the subcontinent of Indoi, and was a meticulous recorder of data and a fastidious observer of details. He had scribed much of the events of the earliest days of the Great Crusade and had been one of the first Remembrancers to be chosen by the Thousand Sons. His reputation had preceded him, and he was immediately assigned to Magnus the Red. He was at Magnus' side from the time the restored XV Legion had departed Prospero to take part in the Great Crusade. Kallimakus served as Scrivener Extraordinary and the personal scribe of Magnus. He was responsible for transcribing the Primarch's own words into a massive tome known as the Book of Magnus. This tome 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 on the Great Crusade.
When the Horus Heresy began, Kallimakus realised his records were a collection of mystic rites now forbidden by the edicts of the Council of Nikaea. Fearing for his safety, he fled with his works before the arrival of the Space Wolves Legion and the Terran Censure Host during the Burning of Prospero.
Following his flight, Kallimakus first sought aid from and was then imprisoned by the nascent Inquisition, as he continued to maintain a psychic connection to Magnus. This psychic connection was transferred by the Inquisition to successive hosts after each died.
Before Kallimakus passed away from old age, he created his life's work, the Athenaeum, using both his own memories and the notes compiled from his time at Magnus' side. The Athenaeum was, in effect, an exact copy of the infamous Book of Magnus and after Kallimakus's death, his work inspired a cult of fanatics to study the Athenaeum for thousands of Terran years, filling up the grand library they established on the world of Apollonia, also named the Athenaeum of Kallimakus, with notes and commentaries upon it.
The location of the Athenaeum, upon the isolated world of Apollonia, was one of the Inquisition's most treasured secrets, but Ahzek Ahriman, the former Chief Librarian of the Thousand Sons, who sought a way to reverse his failed Rubric of Ahriman, ultimately learned of its existence in the 32nd Millennium, from an Inquisitor named Iobel. He kidnapped her, stole the knowledge from her mind and led the former warband of Magistus Amon to successfully assault the Athenaeum and take Kallimakus' works for his own.
Pursued by the Ordo Malleus and its Grey Knights contingent as well as a rogue fleet of the Space Wolves' 13th Great Company, Ahriman arranged for his enemies to fight one another and had the Athenaeum of Kallimakus installed in the body of his friend and betrayer, Sanakht.
- Codex: Chaos Space Marines (4th Edition), pg. 51
- Codex Heretic Astartes - Thousand Sons (8th Edition), "Annals of the Arcane," pg. 28
- A Thousand Sons (Novel) by Graham McNeill, Ch. 26, pp. 164-166, 305, 319
- The Crimson King (Novel) by Graham McNeill, Ch. 12