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-Heret