Prince of the Eye

What if Horus had survived the Heresy ? What if, as the Siege of Terra approached, the Dark Gods conveyed, and changed their plans ? Exiled in the Eye of Terror with the rest of the Traitor Legions, how would the Warmaster adapt to his defeat, and the realities of the Long War ? An AU story in the style of the Roboutian Heresy and the Fifteenth Ascendant.