Ur-Dragon is an online boss in this game. Everyone fighting the boss collectively contributes damage until it is killed, then massive loot drops. Even participating you get something. Each takes a couple of days. Anyway playing bitterback island content. you need to do this before post game. The first time around I did after and it just seemed that the enemies were tremendously harder. I also found some good loot, overall the loot is supposed to be better than Ur-Dragon.