The Horus Heresy: Battle of Tallarn - Ascendance Campaign

Before the crash-landing of 'The Eagle's Talon' unleashed a wave of radiation on Tallarn, while desperate skirmishes played out in the subterranean confines of the Sightless Warren, war raged on the surface still. As word of Tallarn's fate spread further, both sides drew in allies and, with them, war machines of ever greater might and destruction. The forces of the Imperium and the Iron Warriors sought any advantage to push the enemy back and assert their control of Sapphire City, in the hope that dominance on the surface would provide leverage on the labyrinthine tunnels below.

Source: Steam Store Description