The Thallaxii are heavily augmented cyborg shock-troops used by the Mechanicum Ordo Reductor, and they differ from the Skitarii regiments both in purpose and the unique degree of their augmetics. The Lorica Thallax was developed from power armour technology and requires a robust human specimen.
The Lorica encloses the major organs, nervous system and cerebrum, but entirely replaces the skeletal structure and limbs with armoured bionics powered by an internal reactor-core. Surgical excision of the subject’s pain sensors, emotions, and normal human sensory apparatus and the agony caused by the Lorica’s implantation, leaves the Thallax a cold, calculating killing machine (albeit one that retains a degree of independent thought).
To some among the Mechanicum, this operation skirts the edge of abomination, yet the baleful Ordo Reductor continue Thallax conversion regardless; using suitable Forge Guard, fallen Skitarii, and other subjects obtained from more mysterious sources.
(Source: Forge World)