- "Being a full and faithful account of the death of Daemons, Monstruous Prodigies and the Lords of Darkness, and accompanying extrapolations of their return from Banishment."
- —Subtitle of the Codicium Aeternum
The Codicium Aeternum, also sometimes referred to as the Grimoire of True Names, is one of the most valuable and dangerous texts in possession of the Ordo Malleus, the division of the Inquisition that specialises in fighting the daemonic servants of the Ruinous Powers.
The origins of theCodicium Aeternum are shrouded in mystery, and no one truly knows when the titanic endeavour of writing the Codicium Aeternum began. The Codicium contains the information the Ordo Malleus has earned in bloody battle against the daemons of the Warp. It serves as a compendium of the most powerful Greater Daemons thus far encountered by the Inquisitors of the Ordo Malleus and the Space Marines of the Grey Knights Chapter that acts as the Holy Ordo's Chamber Militant. The Codicium focuses primarily on recording each daemon's True Name, as knowing the daemon's true identity considerably weakens the foe in battle. The volume also contains a recounting of the daemon's previous clashes with Imperial forces, the successful rites for its banishment and a determination of the banishment's duration according to a reading of the Emperor's Tarot.
No one truly knows how many copies of the Codicium Aeternum exist or where they all may lie. It comes as no surprise that given the highly sensitive material it contains, only the most experienced and trusted Inquisitors and Lord Inquisitors may consult or even own a copy. The Inquisitorial Archives on Tethys, one of the moons of Saturn that has been claimed by the Ordo Malleus, currently holds a mere two copies of the Codicium Aeternum. The original copy, long thought lost or forgotten as a matter of precaution, was discovered to have been stolen from the archives by the Renegade Inquisitor Gholic Ren-Sar Valinov, an agent of the great Daemon Prince of Tzeentch, Ghargatuloth. Following the banishment of this mighty foe, Inquisitor Nyxos deemed it profitable to manufacture a second copy which consisted of an updated version of the original. This would indicate that several more or less up-to-date variants of the Codicium Aeternum may exist within the Imperium.
The Codicium Aeternum contains much information on the most powerful daemonic entities, including:
The Codicium Aeternum seems to be a mixture of both the Malleus Malificarum ("The Hammer of the Witches"), the 15th-century treatise on witchcraft extensively used to justify countless witch trials in Late Medieval/ Early Modern Europe and later pseudo-scientific publications on demonology and occultism that were quite popular in the 19th century. These included the Dictionnaire Infernal (1818) of Jacques Auguste Simon Collin de Plancy. The Codicium Aeternum's equivalent in the Warhammer Fantasy universe -- The Witchhunter's Handbook -- was published in 2006 by the Black Library.
- Grey Knights (Novel) by Ben Counter