The Liber Mithrus was an ancient tome of knowledge granted to the Imperial Fists Legion at its Founding by the Emperor of Mankind Himself. The Imperial Fists Chapter later inherited the artefact after the Second Founding and held it as a sacred relic, placing it in a special shrine erected on the desolate Shrine World of Mithron. There it was protected by a perpetual Honour Guard composed of an entire company of Space Marines drawn from the Imperial Fists.
During an incursion by the Forces of Chaos led by the Chaos Space Marines of the Black Legion, the book was nearly captured until the surviving Imperial Fists were aided by the arrival of the Ultramarines 2nd Company's Ultima Squad. Unfortunately, a daemon possessed the commander of the 2nd Company, Captain Severus, after the Ultramarines recovered the Liber Mithrus from its shrine.
The daemon intended to use the arcane knowledge contained within the tome to open a Warp portal on the Ultramarines homeworld of Macragge and unleash its daemonic brethren upon that bastion of the Imperium. The daemon was defeated by Battle-Brother Proteus within the Reclusium of the 2nd Company's Battle-Barge by wielding the sacred Hammer of Macragge. It is unknown if the Liber Mithrus was returned to the Imperial Fists following this incident.