This chapter is dedicated to Beast's personal quest.
Stage 1: Fort Joy
The quest starts right after you recruit Beast to your party.
In Arena Of Fort Joy, you can meet Carson who turns out to be Beast's friend. If you let your companion speak with him you can learn that Justinia, the Dwarven queen, runs a secret operation. You will also receive Magister's Key.
Leave Fort Joy Ghetto and go to Sanctuary Of Amadia. There, you can meet Duggan.
Let Beast speak with him. You learn that Lohar knows more about Justinia. He can be found somewhere in Driftwood.
Stage 2: Reaper's Coast
Lohar can be found in Undertavern.
Sadly, he won't talk with you unless you solve his problem with Mordus (see Shadow Over Driftwood quest).
After completing this quest, you learn that Justinia ordered him to bring a Death Fog bomb to Arx.
Stage 3: Arx
Beast can confront Justinia. You can meet her in Isbeil's hideout.
Go to Sewer. There, you can find secret doors that are opened by pressing bricks in a wall. If you have at least 5 Thievery points then you can open them. If not, you can do that by using a key that you can find in a room with spider webs near a Source pond.
There, you learn that the queen was manipulated. This initiates a fight with Isbeil. Win the battle and head to her laboratory where Beast can speak with the queen.