- "I obey no master but myself."
- —The Rogue Thallax Warrior, Sigma XI
Sigma XI is a rogue Thallax that fought on the side of Chaos during the Horus Heresy and now roams the stars as a mercenary and pirate, doing what it must to survive in the Grim Darkness of the 40th Millennium.
Sigma XI leads a small band of fellow Thallaxii, known as the Deathstriders.
The Great Crusade
Sigma XI was one of a handful of Thallax created during the Great Crusade era to further mankind's reach into the stars. It served with distinction in the infamous ranks of the Ordo Reductor, and stood out from the rest of its cyborg brethren in its skills as a leader and combatant. Thus following a successful joint campaign on the mining world of Styxx, Sigma and its Cohort were assigned to the Luna Wolves Space Marine Legion, under the direct command of the Ordo Reductor Magos, Simbali.
Sigma's Cohort would become well known throughout the VII Legion, and it seemed that Horus himself was interested in the killing machine, dubbing Sigmas Cohort 'The Deathstriders' after witnessing them in battle with the Orks of Ullanor.
The Horus Heresy
Unbeknownst to Sigma XI, the Warmaster had become corrupted by the vile influences of the Dark Gods, but Simbali remained loyal. This meant that Sigma and his Cohort were destined to be destroyed alongside the various potentially loyal elements of each Legion on Istavaan III. However, the machine had caught wind of the planned annihilation of all loyalist elements, something within the Thallax reacted to this in a manner no Adept of Mars could have expected... his self preservation systems took hold rather than the need to serve the Omnimessiah. It is said that Sigma marched to Simbali's quarters and tore his head from his body. He then presented the fleshed skull to the Warmaster just days before the massacre took place. Horus, unwilling to trust a machine, but intrigued, assigned one of the Luna Wolves best Techmarines to the Thallax and his Cohort.
Sigma and his fellow killing machines would take part in the purge, slaughtering untold numbers of loyalist Legionaires and some of their fellow Thallaxii who were not wise enough to see the logic, survival depended on service... for now.
Drop Site Massacre, Istavaan I
Sigma XI and his Cohort did not take part in the bloodbath of the Drop Site Massacre, and instead were deployed to Istvaan I with a small Kill-Team of Luna Wolves and Emperor's Children, where rumored Loyalist elements may be hiding. What the machines found were not loyalists, but instead packs of Gene-Stealers! The Emperor's Children were wiped out in the first few minutes as the group made its way through the disheveled mining facilities. The Luna Wolves, nine Terminators and two Techmarines, fought for their lives alongside the Cohort. Sigma XI and his Cohort had secured an exit from the infested facility in the vehicle bays, saving the remaineder of the Kill-Team. Following the scouring of Istavaan I by a larger force, the Cohort was officially assigned its commanding Techmairine, who would command them until the Battle of Terra, Guth Darten.
Sigma XI has no memory of its life before activation as a cybernetic weapon in the forges of Mars, and thus has forever been a cold and ruthless being. Sigma has no emotions, operating purely on logic and for its own benefit. Sigma, unlike so many outcasts from the Imperium, does not hate, or even personally oppose the Imperium's rule. However, as a vast majority of its clientele are heretics, xenos, and other unsavory beings, it has grown accustomed to fighting the very forces who created it.
Sigma XI is armed with a deadly arsenal of wargear, from his built in jump pack, frag grenades, and Djinn Sight, to his Lighting Gun with a large chain bayonet, Sigma is a living weapon and a nigh unstoppable engine of destruction on the field of war.
- "What! No, of course I've never employed the awesome force of mechanized destruction known as Sigma XI and had it destroy my enemies in majestic ways. That would be treason!"