How about a boss where you enter a room full of thrall. A really big room with obstacles and stuff to jump on, and more thrall than the bottom of the hellmouth. In the middle of the thrall is a normal sized, but black thrall that the others swarm around. The black thrall, from now on out called Golgoroth, is in preparation to become an ogre. Essentially, the lights will be out, and to turn them on two people must stand on a plate. The third is free to chase golgoroth, while the other two must defend themselves from thrall. But, those two can only stay on the light for 30 seconds before it goes out. Then, one of the two will be randomly chosen to be a "thrall hunter". The player who was out chasing golgoroth must head to the light now until the next time it is out, and one of the players previously on the light will go to hunt the thrall. The catch is that when a light goes out it is destroyed, and there are only a limited number of lights so you must go fast.