There are many legends whispered among the ponies of Equestria, but none are stranger than that of the metal man that fell from the stars. Legend has it that the metal man's suit of armor still exists to this very day, slumbering the centuries away in the Canterlot Archives ...
Takes place during the season four finale.
Here's some quick background info on what you need to know, seeing as the 40K universe is huge and no one can be expected to know everything.
The Space Marine chapter involved in this story is a little known chapter known as the Celestial Lions, who are a descendant of the Imperial Fists. They were targeted for destruction by the Inquisition after defying them on an Exterminatus that was due to purge a world the Lions had fought for and saved. They fought during the Third War for Armageddon mainly, and during the fighting, they were reduced from 950+ battle brothers to just 96. Their numbers are even lower now, due to events that will be explored during this story.
Edited and Proofread by the amazing Loyalist. He's seen some heresy.
Featured on 4/26/15 and again on 7/25/15