Starting to read the Horus Heresy series from the Black Library.
I'm still only on Horus Rising, it explains how Warmaster Horus and half of the founding legions of Adeptus Astartes fell to the Chaos gods, and how the Emperor of Mankind made the ultimate sacrifice to stop Horus from destroying humanity, in a battle that killed Horus, but left the Emperor on a huge life support machine known as the Golden Throne of Mankind, where for thousands of years he's watched over and guided mankind as it struggles for survival in a universe plagued by constant war. "FOR THE EMPEROR!"