Trade Secrets Quest (Pillars of eternity 2 Deadfire)
You get the quest by Talking to Prince Aruihi in Kahanga Palace.
How to reach Kahanga Palace: https://youtu.be/mUBnpEBekFE
First, find the secret passage to the Delver's Row. For that First Travel to The Gullet. Once You are in the Gullet, talk to Fyrna in The Hole to know the path to Delver's row .
Now That you know the path, go to the Narrows, and follow the path mentioned by Fyrna.
Once you reach the Delver's row, Talk to Ernezzo and show him Envoy's badge. He will ask you to meet Dereo, and tell you the path to Dereo's Lair.
Go and talk to Dereo. Dereo will ask you to bring 'Cornett of waves'. If you do so, you can travel to Undercroft without any resistance.
To know how to get the item check the walkthrough for the Quest, 'The Cornett's Call'. Link is given below.
The Cornett's Call Quest: https://youtu.be/EedFTSvjXp0
Once you have brought the 'Cornett of waves' to Dereo, proceed to Undercroft. To reach Undercroft, you can use the lift in Delver's Row.
Report to Prince Aruihi to complete the Quest.
POE2 Deadfire Game Guide Playlist: https://www.youtube.com/playlist?list=PL_l3PEDN9zYiW3reEcQMv18acyYKWeVMw