The Castellax was the mainstay battle-automaton of the Legio Cybernetica, a hulking robotic war-engine deployed by the Mechanicum throughout the Great Crusade and the Horus Heresy. Squat, heavily armoured and bristling with weapons, it was built to anchor a battle-line, absorbing punishment and returning it with the tireless ferocity that only a machine could sustain.
Unlike a fully sentient artificial intelligence, forbidden by ancient law, the Castellax was governed by a cortex controller and constrained programming that lent it lethal purpose without true will. Under the direction of a Mechanicum adept it would advance, guard or slaughter, executing its directives with pitiless precision.
The robot cohorts of the Mechanicum fielded the Castellax in great numbers, and when the Heresy sundered the Machine God's servants, these engines marched for both loyalist and traitor forge-worlds, turning their guns upon former masters and kin alike.