In an alternate timeline, the Horus Hersey never happened. He and the other 9 primarchs remain loyal to the emperor. But the other nine sons committed heresy by destroying the imperium of man. Led by lion the heresy has shaken the very heavens.

Lemun Russ, sanguineous, vulkcan, Rogel dorn, jagatai khan, robute gillamen, corvous corax, and Ferris manus have turned traitors. Devoted by the chaos gods. Only Horus and the others can save us