Join Date: Mar 2013
Location: Canadian national, US for university
There's two main paths I can see- one, Horus takes over the Crusade (likely passing it off to one of the other primarchs, probably Gulliman or Dorn, one of the 'stable' ones), and returns to Terra. We end up with the Lorgar Heresy and history either plays out identically to the Emperor's own, or Horus wins fully and is hailed as the true successor to the Emperor. The Age of the Imperium begins with a living head and much fewer casualties.
Or, Horus remains on the front lines and is slowly corrupted by Chaos. With no Emperor to stand against him, he easily takes the throne from the few remaining loyalist legions- several 'loyalists' having joined him rather than face annihilation. The human race falls to Chaos, and then whatever the Long War's proposed ending is plays out in M31 onwards.