Random thought about Cain accidently going back in time while in his armor and being mistaken for a primarch.
Excerpts from Primarchs and their affect on history by Inquisitor Murray.
...And so we look at the final and most mysterious primarch, the 11th. In fact the reason why they are so mysterious is do to the simple fact of how few records have survived to the present day. There as so few records that it is acutely easier to count who knows about the 11th than who doesn't. However despite this it can be said that my fellow Inquisitors have benefited the most from the 11th's affect on history, for the agents that we employ ,from the Assassin Temples to the Tempestus Scions, can all trace their origins to the mortals the 11th had personally trained. Yes that is right, the legacy that he left behind is not a chapter of astartes, of which none seem to have survived, but a training regime that is the basis for all specialist agent training……While there are theories about what happened to the 11th, the most compelling theory is based on how the records of the 11th suddenly stop a few weeks before the traitor Horus began his heresy. With the only piece ever found being his horned helmet it is believed the Horus removed the 11th in a attempt to make there rebellion more likely to succeed…