Liber Mithros
The 'Liber Mithros' is the name given to an ancient codex of dark power given to the care of the Imperial Fists Chapter.[1]
Overview
According to Chaplain Carnak of the Imperial Fists 5th Company, the Emperor himself gifted the book to the chapter at its inception, making it at least over 10,000 years old. A shrine to house the tome was constructed on the world of Mithron, protected by an entire company of Imperial Fists with attending Imperial Guard units.[1]
In the 40th Millennium, the shrine came under a vicious attack by Black Legion traitor space marines, whom the Imperial Fists and Guardsmen managed to hold at bay for three weeks while calling for reinforcements. Then, a warp portal opened in the sky, unleashing a horde of daemons that massacred almost the entire garrison, desecrating the corpses thereafter. By the time Ultima Squad under Captain Severus of the Ultramarines arrived, only Carnak and brother Nidon, who was charged with personally carrying and protecting the book, remained. Surviving another attack from the Black Legion, the two Fists and remaining Ultramarines evacuated to an orbiting Ultramarine Strike Cruiser. When Severus was revealed to have been possessed by a daemon himself, he killed most of the marines, including Carnak, and took the book to the ship's reliquary with the plan of creating a portal over the Ultramarine homeworld of Macragge to grant the planet a similarly grisly fate to that of the Mithron shrine. This plan was narrowly foiled by Ultramarine brothers Proteus and Verenor shortly before the ship exited the Immaterium above Macragge; what became of the book afterwards is unknown.[1]
The book's pages appear to be blank when dormant but, with dark incantations, writing will begin to appear in the pages that seem to allow the user to create a warp portal at the desired location.[1]
