Rubric Marines

Rubric Marines are the remnants of Astartes warriors of the Thousands Sons Legion that were inadvertently created when the Chaos Sorcerer Ahriman cast an incredibly potent spell called the Rubric of Ahriman to halt the process of degeneration and mutation caused by the energy of the Empyrean that afflicted his battle-brothers after they had escaped to the Eye of Terror and were exiled to the Planet of Sorcerers. Though their corruption had been halted and they were now devoid of mutation, they were all but stripped of independent will as well.

History
Following the Burning of Prospero during the opening days of the Horus Heresy, the Thousand Sons settled upon a world prepared for them by their patron Chaos God Tzeentch, the Changer of Ways, upon the Planet of Sorcerers deep within the Eye of Terror. Given the heritage of their Primarch, it can be no surprise that the Thousand Sons Legion was always prone to mutation. Prior to the Heresy, stringent purity checks and relentless discipline had kept mutation at bay whilst simultaneously developing psychic power. Having steeped themselves in the raw power of Chaos, however, the Thousand Sons fell prey to rampant and uncontrolled mutation they called the "flesh change," to such an extent that a cabal of the Legion’s senior Librarians, led by their Chief Librarian, Ahriman, determined that something drastic must be done to save the Legion from complete dissolution. With Magnus retired to the highest tower of the City of Light, his mystic, all-seeing gaze cast bitterly upon the dimensions without, Ahriman and his cabal set about enacting a mighty spell that would purge the Legion of mutation and impurity, and leave behind a body of utterly purified warriors.

Ahriman delved deep into the pages of the Book of Magnus, a massive tome compiled by the Legion's Primarch Magnus the Red, that was filled with forbidden lore and knowledge from ancient, forgotten days. Ahriman believed the Book of Magnus held the key to his Legion's salvation. In the labyrinthine collections of formulae, incantations and rites, Ahriman devised what he believed would be the beginnings of a mighty arcane spell to undo all that had befallen his Battle-Brothers, known as the Rubric of Ahriman. Ahriman and his cabal cast his Rubric, and the skies over the Planet of Sorcerers erupted in an etheric storm of unprecedented proportions. Bolts of power formed from the raw stuff of the Warp arced from the roiling clouds, each striking a Battle-Brother of the Thousand Sons, until all but Magnus in his tower and the cabal of Sorcerers had been scoured by the mighty rubric. But when the storm receded, Ahriman saw the awful truth of what he had wrought. Instead of purging the flesh of ravening mutations, each of the Thousand Sons had been transformed. The seals and joints of their Power Armour had been welded shut, and the body within turned to ashes. What remained was a suit of animated armour, devoid of mutation, but of all independent will as well.

The newly created Rubric Marines, were living suits which still moved and functioned, and could respond to orders just like a sentient man, though they were now little more than automatons. They quickly fell into inactivity unless a Thousand Sons Chaos Sorcerer is nearby to direct them, although in the fire of combat something of their former battle hunger returns and they move with greater clarity and purpose. The Thousand Sons Sorcerers use their Rubric Marine brethren as bodyguards and enforcers as well as guardians for the great libraries of grimoires and vaults of ancient scrolls held by the sorcerers. The Rubric Marines are excellent in these regards, for they have no spark of curiosity left and are utterly loyal. If promised with knowledge and magical power the Sorcerers are willing to offer the services of their unliving warriors. In battle, the Thousand Sons Sorcerers enchant their weapons and those of their unliving bodyguards, their bolts blazing with baleful sorcerous energies and exploding with sorcerous blasts that burn the souls of their targets as much as they do physical damage.