I totally misunderstood the title. I was expecting it to be something more along the lines of a Notorious Monster, but in the form of a dungeon. It would be a highly specialized dungeon that would only randomly pop up during a duty roulette and would have extra cool rewards in the chests or at the end or something. Maybe they don't even need to be full length. Maybe they could be mini dungeons that just happen at random from doing the roulette...