The Malus Codicium is a dangerous tome of daemonic knowledge. It was highly sought after by several individuals, most notably amongst whom were Inquisitor Helgrund, who sought to destroy it, and Pontius Glaw, who required its vast sum of knowledge to acquire the Daemon-king Yssarile's barque for himself.
It was in the possession of the renegade Inquisitor Quixos until he was killed by Gregor Eisenhorn. Instead of handing the book over to the Inquisition, Eisenhorn hid it for his own use, and through it learned how to bind daemonhosts and create psychic thralls.
The book was finally destroyed in a confrontation between Eisenhorn and Pontius Glaw on the planet Ghül.