Summary
The Imperial priesthood of Mars — keepers of technology, worshippers of the Omnissiah, and forgers of the Imperium’s war materiel.
Lore
The Adeptus Mechanicus is the Imperial priesthood of technology, headquartered upon the forge world of Mars — Forge Primus. Its Tech-Priest adepts believe that knowledge is the manifest divinity of the Omnissiah, and that all machines possess a Machine Spirit worthy of veneration. The Cult Mechanicus rose during the Age of Strife on Mars, surviving the long isolation that severed it from Terra, and re-entered the nascent Imperium as a sovereign power through the Treaty of Mars sworn between the Emperor and the Fabricator-General.
Under that ancient covenant the Adeptus Mechanicus retains its own creed, its own armies, and its own forge worlds across the galaxy, while supplying the Imperium with the arms, vehicles, void ships, and cogitators that sustain its survival. Its members range from humble enginseers attached to Imperial Guard regiments to Archmagi Dominus who command entire war hosts of Skitarii and Legio Cybernetica battle-automata.
Doctrine
The faith of the Cult Mechanicus is codified in the The Sixteen Universal Laws — the Universal Laws of the Mechanicus — and animated by the The Quest for Knowledge, the eternal hunt for lost archeotech and surviving Standard Template Construct (STC) fragments. The Tech-Priests believe the Omnissiah is the divine spirit of the Machine God, with the Emperor of Mankind venerated as its mortal incarnation; this dual reverence is the doctrinal bridge that holds the Treaty of Mars together.
Hierarchy ascends from acolyte enginseers through Magi to the Archmagos and ultimately the Fabricator-General of Mars. Augmentation of flesh with the sacred machine is itself an act of devotion: a Tech-Priest’s standing in the cult is read in the extent to which they have shed weak biology for blessed cybernetics.
Military
The Mechanicus fields its own armed forces independent of the Astra Militarum. The Skitarii legions form its line infantry — cybernetically-augmented warriors raised on forge worlds and slaved to the will of their controlling Magi. Alongside them march the Legio Cybernetica cohorts of Kastelan robots and Kataphron battle-servitors, the demolition-engineers of the Ordo Reductor, and the war-engines of the Centurio Ordinatus.
In the high orders of war the Adeptus Mechanicus deploys Titans of the Collegia Titanica and the Knight Houses bound to forges such as Lucius — Forge of the Knight Houses. Its archmagi, including Belisarius Cawl of Mars, have authored some of the most consequential military and biological reforms of the late M41 and M42 — the Primaris programme chief among them.