1. The Black Bull - Undertavern entrance
2. Driftwood - Mordus' house
3. Reaper's Coast - The Wreckers Cave entrance
Speak with Lohar in Undertavern . He wants you to go to Mordus' house and search it.
Mordus' house  is located next to The Black Bull. Inside of the building, you can find one of Lohar's people. Speak with her. After that she will discover a hatch that leads to basement. There, you can find a room with imprisoned dwarves. Free them by using the lever on the wall. Speak with them and start searching the room. You can find a trap that activates when you try to get closer to the chests. Because of that, you should leave one of your team members outside so they can open them again. Behind two chests you can find a hidden bookshelf. There, you can find a message from which you learn that Mordus wants Lohar to be dead.
Go back to Lohar and inform him about your discovery. He wants you to kill Mordus.
Mordus can be found in The Wreckers Cave . You reach a cave full of corpses. You will also be attacked by a Voidwoken. This isn't a regular fight because, after each turn, one of your team members will be transported to a different part of the cave. This means that you team is completely scattered.
The best option is to start playing as a character that is located in a room that is similar to a bedroom. That character has to fight with a rather small number of enemies and can find a key that opens doors in the cave which makes your task of completing the team easier.
When your team is complete, you can go to a shipwreck where you can find a ladder that can take you on the upper level.
Warning: You can also order one of your team members to leave and use Teleporter Pyramid. If you teleport your entire team on the beach that is next to the shipwreck and enter The Wreckers Cave from this side then you will be able to face Mordus immediately.
Now, you must face Mordus. Defeat his possessed form. He stops fighting and asks you for your mercy. You can agree to that or kill him. Regardless of your choice, you get Mordus' Amulet.
Go back to Lohar and inform him about your actions. He will offer you a key to a chest located in magisters' building.
Reward: 4 guaranteed rewards + 1 from 4 that you can choose from depending on character's class