It is believed that the first Primarch to fall from the Emperor's light was Lorgar of the Word Bearers, who then proceeded to corrupt several of his brothers-including Horus Lupercal himself-leading to the Horus Heresy.
This information was false. Another Primarch had fallen once before and the fall of one Legion led to this Legion's existence and that of one other to be completely expunged from Imperial records.

The Forgotten and The Purged, The Second and The Eleventh, the first two Astartes Legions to face off in an all out war-a war that Equestria was dragged into. Will the Legiones Astartes be willing and/or able to protect this new world as they would their own? Or would Chaos consume this realm through the new crack in reality?

This story is on a 'trail run' sort of thing. I wrote these first two chapters to see how the idea would be received. I may continue if I get to a place in my other story where I feel I can leave it for a bit, or if this is asked for enough - not that I expect it will be Call me a realist.