Serving Bregan D'aerthe is a Side Quest in Sword Coast Legends. You can start this quest by following the informant after meeting Sorenil in the basement of the Cutlass in act II.
- Mysterious Stranger
- Recover Bertrand Lucere's Abyssal Relic
- Recover Ship Rethor's Abyssal Relic
- Report your findings to Bregan D'aerthe
- Pursue the imposters further into the sewers
After a short cutscene and dialogue, locate the three hidden switches on the walls in the storage room behind Sorenil, and activate the lever in the middle of the room.
Enter the abandoned sewers, and makes your way down the left passage.
A cutscene will trigger that shows the Mysterious Stranger locking a door behind him.
Head to the next room, defeat the black pudding, and locate a series of secrets doors to get around the locked door.
Speak with the Mysterious Stranger.
Complete the Following Quests:
The Other Luskan Cult
A Trader's Fate
Return to the Mysterious Stranger.
Upon returning the abyssal relics to the Mysterious Stranger, you'll come to the sudden realization that you've been a pawn in bigger scheme.
You will be attacked by a small horde of devils. Defeat them, and pursue the Priest of Hethradiah and the Priestess of Aameul further into the sewers.
FUN FACT: Each of Demogorgon's heads had its own name. One was Aameul while the other was named Hethradiah. The two heads warred with each other. Aameul relished deception, while Hethradiah craved destruction.
Continue down the path and speak to the "real" Mysterious Stranger. He will tell you that the two who deceived you are planning to open a portal to the Gaping Maw, the 88th level of the Abyss and home of the Demogorgon.
Enter the next room, and confront the cultists. Once they are defeated, speak with the Mysterious Stranger again. He will instruct you to meet him in your "usual spot".
Simply go back to the previous map (avoid the world travel in the boss room for now), and speak with the Mysterious Stranger again.
If you go into the conversation with 24+ Charisma score, you can learn the Mysterious Stranger's true name.