Each Rubric Marine is the terrible legacy of the ritual Ahzek Ahriman wrought to save his Legion from the flesh-change. Where the spell spared and strengthened the strongest sorcerers, it hollowed out the many thousands who lacked such power, reducing them to nothing but animate dust bound within their ceramite shells. They retain no memory, no will and no soul, only an eerie, mechanical existence and an unbreakable obedience to their masters.
Upon the battlefield they are a chilling sight, advancing in perfect, silent unison with no fear to check them and no pain to slow them. Their inferno bolts, blessed with the changing fire of Tzeentch, sear through armour that would turn aside common munitions, and they march through withering fire without falter, for there is nothing left in them to break.
Yet a Rubricae is nothing without a sorcerer to guide it. Left without command, the dust within the armour grows still, awaiting orders that may never come. In this the Rubric Marines embody the whole tragedy of the Thousand Sons: warriors who sought knowledge and found only oblivion, now marched to endless war as the tools of the very ambition that destroyed them.