Doctor's Order quest description:
Stage 1: Reaper's Coast
During your stay on Bloodmoon Island you will be able to speak with Ancestor Tree. She will tell you her story. She also asks you to kill the demon that imprisoned her. The quest activates when you agree to fulfill her request.
Stage 2: Arx
The demon is called Adramahlikh and he hiding as Doctor in The Black House.
Killing him is very difficult, mainly because you will have to face everyone that is in the building. A good solution for that is to kill all characters that are in the rooms that are closer to the exit. You can't allow the Doctor to join in. If you don't kill these characters, they will join the fight with the Doctor right after you initiate it.
The quest ends with Adramahlikh's death.
Reward: 137,450 experience points