Go to Barracks and speak with Linder Kemm.
You learn that the paladins are searching for the remaining magisters. You can join them in their searches.
The entrance is located in Barracks in a room that looks like a warehouse. There, you can find a hatch that can be opened after saying certain words in the correct order. These words are the magisters' motto. Select the dialog lines that refer to: Mind, Body, Society, Loyalty.
This allows you to go to the undergrounds which serves as magisters' hideout. However, your target is located one floor below. There, you can find magister Reimond and his Gheists. Kill them and go back to Linder Kemm to collect your reward.
Reward: 426,300 experience points + 4 guaranteed rewards + 1 out of 4 to choose from depending on the character's class