So, what if the wyvern he is riding IS Ornstein? His armor is found without a corpse, suggesting it was set aside because it wasn't needed anymore. My headcanon is something like this. In DS1 you get to see that Ornstein survived before Smough smashed him, so if instead you do the fight in the opposite and take down Smough first, its possible that Ornstein could have survived in that powered up form. He was devoted to the Firstborn, and after his defeat went searching for him again. In DS2 I believe the "ornstein" we fight here is simply a set of similar dragonslayer armor being controlled by Nashandra, hence the dark based attacks. It could possibly be a version of him corrupted by the abyss during his search for Faraam, after having heard of the lion-knights who worship him; however I don't see how he would survive us here if it were actually him. By the time DS3 rolls around, he has found Faraam who has sacrificed everything to ally himself with the dragons. Ornstein, in his quest of returning to serve him, begins to follow the Path of the Dragon to atone. The miracle lightning storm says that Faraam tamed a Stormdrake and led it in a "lifetime" of battles, forming a bond. It is possible that the Stormdrake was killed eventually in one of these battles, and that Faraam kept his soul as the gods are want to do. We've seen how powerful souls can physically transform characters, so what if Faraam blessed Ornstein with the soul of the Stormdrake, turning him into the "King of Storms".