Horus Heresy is civil war on a galactic scale, a conflict comprising countless bitter campaigns and doomed struggles. In the latter years of the Age of Darkness, the Loyalist forces occupying the Sons of Horus home world of Cthonia face a determined assault from overwhelming Traitor forces, who seek to oust them from the ancestral home of the Warmaster Horus. The resulting war of attrition rages in the skies above and the tunnels below the surface of that embattled world, taking a terrible toll on Loyalist and Traitor alike.