The Legio Cybernetica is among the oldest and most jealously guarded institutions of the Adeptus Mechanicus, a martial order that constructs and commands battle-automata: towering robots of iron and adamantium that know neither fear nor mercy. Its origins reach back to the Dark Age of Technology, when thinking machines nearly unmade humanity, and so its every construct is shackled by cortex programming lest the abominable intelligence of the Men of Iron ever wake again.
At the heart of each cohort walks the Magos Dominus, a tech-priest of the highest rites who alone may inscribe the wisdom-scrolls that govern an automaton's behaviour. The robots themselves are not truly alive, nor truly machine, but somewhere between: bound engines that follow rigid doctrina wafers, striding into battle to crush the enemies of the Omnissiah beneath piston-driven fists and volleys of ancient weaponry.
The Legio's knowledge is fragmentary, its greatest constructs lost to time and the failure of memory. Each surviving pattern is a relic beyond price, rebuilt from half-understood STC data and guarded against the covetous. To lose an automaton in battle is a wound that may never heal, and so the tech-priests hoard their war-engines like misers, unleashing them only when the survival of a forge world or the honour of the Machine God demands it.