From the website:
The Adeptus Mechanicus are the keepers and manufacturers of technology for the Imperium of Mankind. It is a role they guard jealously, using their own standing armies, engines of war and god-machines. It is within their power to unleash the most arcane weaponry in the Imperium, and they do so without mercy. Grim processions of soldiers stride in perfect unison, each one a disturbing fusion of man and machine. Behind them stomp soulless automata, and towering over them are enormous walkers. An Adeptus Mechanicus army marching to war is a sight both terrifying and glorious.