Horus Heresy

The Horus Heresy was a civil war fought in the 31st Millenium in the Warhammer 40,000 setting between the Emperor's loyalists and those led by the Chaos-possessed Primarch, Horus. The conflict started with the virus bombing of Isstvan III and ended with the Battle of Terra and the death of Horus, as well as the ascension of the Emperor to the Golden Throne. After the Battle of Terra, the chaos-corrupted forces fractured and retreated to the Eye of Terror.