There is a school in Arx that is occupied by Sister Schori. Speak with Beryl Griff.
Convince her that you want to help. She asks you to get rid of Sister Schori and make her leave the school's building.
Warning: Beryl is Griff's wife, you met him on Fort Joy. You can receive additional 19,400 experience points if you tell the woman about her husband, regardless of whether this information is true or not.
After that you should go to the school. In order to open the door, you will need a certain amount of points in Thievery. Then, you must speak with Sister Schori and convince her to leave the building.
After that you can go back to Beryl and collect your reward.
Reward: 96,650 experience points + 2 guaranteed rewards + 1 out of 4 to choose from depending on the character's class