The Thallax Cohort was a formation of the Mechanicum's cybernetic warriors, deployed in great numbers during the Great Crusade and the Horus Heresy. Each Thallax was a heavily augmented soldier whose surviving brain and nervous tissue were sealed within a bulky, ambulatory war-frame, blurring the line between man and machine.
Thallax troops were faster and more resilient than baseline infantry, capable of short bursts of flight via integrated jump systems and armed with lightning guns, multi-meltas, or photon thrusters. They served as skirmishers and shock-troops for the Taghmata armies of the Mechanicum, filling the tactical space between mortal Skitarii and towering battle-automata.
When the Heresy split the Priesthood of Mars, Thallax Cohorts fought on both sides of the conflict, their absolute obedience making them ideal instruments for whichever faction commanded their cohort. Their unfeeling advance embodied the cold, relentless character of Mechanicum warfare.