Well, i saw him outside bloated man's grotto after Ill Met by Moonlight quest, i immediately thought: "Another blood dragon soul" so i called Odahviing and Durnehviir to my aid so i was surprised when my arrow hit him, it was named Vuljotnaak. I used Bend Will shout on him and rode him. I noticed on this page that "he will may never be encountered after Alduin is defeated." I'm already done with Dragonslayer so, what? is this like a bug or just some lucky coincidence?