The third crypt contains a possessed Godwoken - Rajjarima.
In order to reach that room, you must fix a lever on the door. Use Sliver Lever Shaft. However, it turns out that you can't open it.
Use Spirit Vision and speak with Brother Robert's ghost. You can convince him to let you in or you can use Source Vampirism. Regardless of your decision, the door will open.
In the next room you must fight with Rajjarima. If you managed to convince Brother Robert to let you in then you will be able to speak with the possessed first. The best option is to enter the room with one character before the fight takes place. Rajjarima has a powerful ability called Arrow Storm which she usually uses in the first turn. If your team members are standing close to each other then she can kill some of them right at the beginning of the fight. If you managed to survive this attack then the rest of the fight shouldn't be a problem to you.
Reward: 57,775 experience points