Ohmn-Mat is a former vassal Forge World of the ancient Mechanicum that was destroyed during the Horus Heresy, when the planet was consumed by civil war. Reduced to a cinder, the fires of Ohmn-Mat continue to rage to the present day.
The early history of this obscure Forge World has been lost down the long millennia. With the outbreak of the galaxy-wide civil war known as the Horus Heresy, the Forge World of Ohmn-Mat fell into a similar state of fratricidal conflict as its Synod of Magi was split in its support for the Loyalist and Traitor causes, drawing in contingents of a Sons of Horus Dark Compliance force and Iron Hands Istvaan V survivors as the conflict quickly escalated. In circumstances which remain unknown, the planet's atmosphere was somehow ignited, shrouding the world in fire. Secondary atomantic explosions ravaged the surface as forge-fanes across the world succumbed and fuelled the flames. Ohmn-Mat was destroyed and its fires still rage in the present day.
- The Horus Heresy - Book Four: Conquest (Forge World Series) by Alan Bligh, pp. 29-30