For a written guide:
1. After the fourth boss battle, travel to Darsov, and exit to the left of the city.
2. A monk will stop you, asking you to return to the cathedral. Oblige.
3. Enter the door to the cathedral, and then the second similar looking doorway.
4. Start the mission in that room by talking to the priest.
5. Teleport to Arkos, and travel to Tohan Passage.
6. Go to the bottom right and enter the church/cathedral there.
7. The man will automatically be excited to do the exorcism and teleport you back with him.
8. Watch the video/follow the dialogue. :)