Sanguinius: The Great Angel by Chris Wraight.

Rogal Dorn: The Emperor's Crusader by Gav Thorpe.

Mortarion: The Pale King by David Annandale.

Alpharius: Head of the Hydra by Mike Brooks.

Lion El'Jonson: Lord of the First by David Guymer.

Konrad Curze: The Night Haunter by Guy Haley.

Vulkan: Lord of Drakes by David Annandale.

Jaghatai Khan: Warhawk of Chogoris by Chris Wraight.

Ferrus Manus: The Gorgon of Medusa by David Guymer.

Fulgrim: The Palatine Phoenix by Josh Reynolds.

Lorgar: Bearer of the Word by Gav Thorpe.

Perturabo: The Hammer of Olympia by Guy Haley.

Magnus the Red: Master of Prospero by Graham McNeill.

Leman Russ: The Great Wolf by Chris Wraight.

Roboute Guilliman: Lord of Ultramar by David Annandale.

The Master of Mankind by Aaron Dembski-Bowden.

The Damnation of Pythos by David Annandale.

The First Heretic by Aaron Dembski-Bowden.

The Flight of the Eisenstein by James Swallow.

Beyond the core conflict with the legions of Space Marines, their Primarchs, and the Emperor, the narrative splits into various sidestories fleshing out virtually every major aspect of humanity's civil war, and the events preceding it. Long established in the background as being directly responsible for the shape the galaxy is in in the 41st millennium, it is the subject of the Horus Heresy series of novels and audio books, by various authors.

The God-Emperor was permanently injured in a lethal duel with their leader, the Primarch Horus (his son, in a way), to the point that he is only kept alive by an extremely complex life support device known as the Golden Throne. In the Backstory of the Warhammer 40,000 universe is humanity's era of hope, only just starting to dawn after the long and terrible Dark Age, before it is brought to an end by the Horus Heresy, wherein fully half of the best warriors the human race had to offer turned to worshipping dark gods and nearly wiped out the other half.

1d4chan horus heresy