The Athenaeum of Kallimakus was a great subterranean library and repository of forgotten knowledge located on the world of Appollonia, the moon of a gas giant and the location of a secret fortress of the Imperial Inquisition.
The Athenaeum was maintained by a cult of savants that worshiped the memory and works of Mahavastu Kallimakus, the Remembrancer who served as the personal scribe of Magnus the Red, Primarch of the Thousand Sons Legion, during the Great Crusade.
Sixteen hundred years after the end of the Horus Heresy in the seventh century of the 32nd Millennium, when he was exiled from the Thousand Sons for casting the Rubric of Ahriman, Ahzek Ahriman led a warband of Rubric Marines to Appollonia after learning of the Athenaeum's existence.
After a month-long siege defended by the forces of the Inquisition and the Grey Knights, Ahriman and his Heretic Astartes broke into the Athenaeum and looted the tomes of knowledge therein in search of a cure for the Rubric. When none was found, they burned the library to ashes, to ensure that no one else would ever learn its secrets about their Primarch and his Legion or be able to use the knowledge of the Warp contained within.
After Ahriman was successful in penetrating the Athenaeum, his destruction of the massive subterranean complex fatally weakened the physical cohesiveness of Apollonia. A short time later, the moon imploded, the debris swallowed by a Warp Storm that was unleashed as the world was destroyed.
In the wake of the Battle of Prospero at the start of the Horus Heresy, the Remembrancer Mahavastu Kallimakus, the personal scribe of Magnus the Red, fled that dying world along with many of the works he had written based on the dictations of the Thousand Sons Primarch.
Kallimakus ultimately sought aid from the Inquisition after the Heresy. Kallimakus hoped to establish a place where the forbidden and heretical knowledge of Chaos and Warp lore that had been gained by Magnus could be both studied by Imperial savants but also kept out of the hands of those who would misuse it.
The early Inquisition proved receptive and offered him the secluded moon of Appollonia to establish his repository. Before Kallimakus died peacefully of old age, he built the Athenaeum, compiling its archive of heretical knowledge both from, his own memory and the notes of Magnus' dictations to him.
The Athenaeum thus housed an almost exact copy of the information contained within the infamous heretical work known as the Book of Magnus.
After Kallimakus' death, this knowledge led to the creation of a fanatical cult of Imperial savants who studied the knowledge kept within the Athenaeum for over a thousand Terran years. These scholars added to Kallimakus' work and filled the library with tens of thousands of new works, much of it speculation and commentaries on the Remembrancer's original body of knowledge.
Fall of the Athenaeum
After the Thousand Sons fled Prospero into the Warp and made a new home on the Planet of the Sorcerers, they were once more afflicted by the ruinous mutation known as the "Flesh Change." To stop it, Ahzek Ahriman and his cabal of like-minded sorcerers sought to enact the great spell known as the Rubric of Ahriman.
The Rubric succeeded, but at the price of transforming the majority of the Thounsand Sons battle-brothers into the undying automatons devoid of all intelligence and personality known as Rubric Marines.
Ahriman was exiled along with many of his fellow sorcerers from the Thousand Sons by Magnus the Red in the Primarch's anger at the outcome of the Rubric.
In desperation, Ahriman recalled the work of Mahavastu Kallimakus who had recorded much of Magnus' research and began to search for the Remembrancer. The sorcerer hoped to read the exact words and procedures he had used to cast the Rubric from a copy of the Book of Magnus once more, in the hopes that a new analysis could lead him to detect the flaw that had produced the Rubric Marines. Ahriman believed he could reverse it and restore his brothers to full life.
Though it supposedly took him 1600 Terran years, Ahriman eventually discovered Kallimakus' fate and the existence and location of the Athenaeum from the mind of the captured Inquisitor Iobel, who had been sent into the Eye of Terror on a mission to determine what the Thousands Sons were up to after the Horus Heresy.
Ahriman immediately took his warband to Appollonia after learning of the existence of the Athenaeum from his interrogation of Iobel. He was opposed by the forces of the Inquisition and the Grey Knights present at the Inquisitorial fortress on the moon and even elements of his own warband turned on him. The resulting battle produced heavy losses on all sides, but Ahriman ultimately secured the library.
After he proved unable to determine the flaw in his spell-casting, Ahriman burned the library to ashes so that no others could gain the knowledge of Magnus the Red. He left the world behind, to continue his quest for redemption and knowledge elsewhere.
The Athenaeum had been hidden deep underground, near to the core of the moon, and its structure now largely held the small world together. Once it was destroyed, the moon soon after collapse in upon itself even as its debris was swallowed by a Warp Storm.
- A Thousand Sons (Novel) by Graham McNeill, Ch. 9, pp. 164–166
- Ahriman: Sorcerer (Novel) by John French, Chs. XIII, XVI, XXI
- Codex: Chaos Space Marines (4th Edition), pg. 51
- Codex: Chaos Space Marines (6th Edition), pg. 60