The Rubric of Ahriman: Ahriman was the greatest of the Thousand Sons Space Marine Legion's Librarians. He retained his premier position as its greatest sorceror bar Magnus the Red when the legion turned to Chaos. The great spell known now as the Rubric of Ahriman was his attempt to protect the Legion from the physical mutations of Chaos. Working with the other sorcerors of the Legion, Ahriman unleashed vast magical energies; shimmering clouds of magic enveloped the Planet of the Sorcerors and great bolts of gold and blue lightning arced from these, striking the Thousand Sons in turn. No sorcerous ward or physical armour was able to deflect its power.
The storm of magica energies might have raged on for days or centuries had Magnus the Red not brought it to an end. In the aftermath, it became that Ahriman had both succeeded beyond all expectation and failed utterly. The Sorcerors of the Legion had either survived and been blessed with greatly augmented powers and knowledge, or been blasted into oblivion.
Yet it was upon the Battle Brothers of the Legion, those with no psychic powers, that the Rubric had its most disastrous effect. The legionnaires were now safe from the mutating effects of the Warp; with no physical body to mutate...Their armour had been sealed beyond all attempts to open it, and their bodies reduced to dust within. Their spirits remained, sealed in their power armour as within plasteel, adamantium and ceramite sarcophagi. Whilst the Rubric protects them form mutation, and renders them almost impossible to kill, it also removes much of their will and vitality. The brethren are little more than animate armour, and swiftly fall into disuse unless a Sorceror is present to direct them on some task.
Source: Horus Heresy: False Gods by Dan Abnett, Codex: Chaos, 2nd Edition by various, Codex: Chaos Space Marines, 3rd Edition by various
Submitter: Simguinus
Hive World Terra