The Malus Codicium was a proscribed grimoire that detailed a large number of powerful sorcerous rituals. The book itself was said to be thousands of Terran years old and in the wrong hands its knowledge could be used to devastate entire worlds. It was sought after by a great many hands over the years, including Inquisitor Helgrund who sought to destroy the book, and the corrupted heretical noble Pontius Glaw of the world of Gudrun, a follower of Slaanesh, who sought to use it to acquire the barque of the Daemon-King Yssarile. The text finally fell into the hands of the Radical Inquisitor Gregor Eisenhorn, who used the knowledge within to serve the Imperium at an increasing cost to the purity of his own soul before it was destroyed during a confrontation between Eisenhorn and the Arch-Heretic Pontius Glaw on the world of Ghul.
Like most occult texts of Chaotic lore, the Malus Codicium's true origin is shrouded in mystery. Its existence and the great danger contained within its pages was apparently known in quite wide circles within the Imperium of Man, since the Inquisitor Helgrund chose to spend the first three decades of his life finding it. When he finally managed to trace the book to the library of Othella on Zandrini Prime it had already been stolen by the Mystic Path, a cabal of dabblers in Warp sorcery in the service of the Radical Inquisitor Quixos. In the following confrontation, Quixos destroyed Helgrund utterly, and took the first of his many steps along the road of damnation.
With the grimoire and his other proscribed sources of heretical information, Quixos came to the conclusion that by recreating one of the Necron-built Cadian Pylons on the planet of Farness Beta, he could “pacify” the Warp and wipe the Eye of Terror permanently from existence. Since the Pylon's original purpose was to separate realspace permanently from the Immaterium this attempt would likely have resulted in the destruction of all souls in the Cadia System.
Quixos' plan was foiled by the Inquisitor Gregor Eisenhorn, who killed his rogue colleague and took the Malus Codicium for his own. With the knowledge he gained from the book he bound Quixos' daemonic servant Cherubael in a dungeon on Gudrun, as punishment for the daemon's attempts to manipulate Eisenhorn. Eisenhorn spent much of the following decades studying the cursed book and in time he came to see it as his only chance to defeat his enemies. With the book as his teacher, Eisenhorn learned how to create daemonhosts, undead thralls and explosive Warp vortexes among other dark secrets. In the end, the now Radical Inquisitor destroyed the Codicium during his final battle with the Heretic Pontius Glaw on the world of Ghul, but by that time he already knew much of the text's arcane secrets by heart. This dark knowledge came to be his main tool to defeat the enemies of the Emperor and the daemon Cherubael became his only ally.
- Eisenhorn Trilogy by Dan Abnett:
- Xenos (Novel)
- Malleus (Novel)
- Hereticus (Novel)