Document the initial attacks during the Schism of Mars in The Rise Of The Dark Mechanicum - Legions Imperialis, the newest expansion for the Horus Heresy. This comprehensive add-on introduces both Loyalist and Traitor factions of the Mechanicum to the large-scale warfare of the era, as the Dark Mechanicum pledges their allegiance to Horus and utilizes their deceptive technology in combat.

The narrative and missions included in this book centre around the Doom of Magma City, a sprawling conflict that set the stage for the Mechanicum’s wider role in the Heresy. Linked missions and new rules for an Escalation Campaign – which tasks players with building up their forces before culminating in a final do-or-die battle – appear alongside army lists for Mechanicum Taghmata and Dark Mechanicum forces, as well as full rules for fielding armies entirely composed of Titans and Questoris Knights.