Cult Mechanicum represents the technocratic might of Mars during the Horus Heresy. Unlike the Legiones Astartes, they are not Space Marines but cybernetic priests, automata, and war machines who worship the Omnissiah – the divine principle of the machine. They are divided into several sub-factions: Taghmata, Cybernetica, Reductor, and more.

During the Heresy, the Mechanicum split – some remained loyal to the Emperor, while others joined Horus and became the foundation of the Dark Mechanicum. The conflict on Mars was one of the first acts of civil war. Their units include Thallax Cohorts, Castellax, Thanatar Siege-Automata, Tech-Priests, and Archmagos.

In the Horus Heresy system, the Cult Mechanicum has access to rules like Cybertheurgy, Orders of High Techno-Arcana, and Advanced Reactions such as Scornful Fire. Their combat style emphasizes heavy firepower, resilience, and technological superiority. They are the first non-marine army with full rules in the new edition.