Just won the game with the Quest of Mastery by killing this dude. Fight was anything but easy, and yes the dude is mounted on a dragon. May I suggest that after you win the game that it allows you to keep playing, if you wish, with all the fat loots dropped off this guy. Including, but not limited to the dragon mount + the staff of domination. Think that would be pretty cool. Kind of a anti-climactic end after this battle the game just ends. I should note that I think I had the quest of mastery victory condition turned off. There was also a weird glitch type where my sovereign was dodging all of this dudes dragons attacks.