How to unlock: Investigate the huge crystal found in Valley of Dead Heroes (M12.14).
Note - Proceed with this task before you embark to The Endless Gate to complete Endless Horror. Otherwise, the quest will be canceled and you won't be able to do it.
To begin this quest, find a huge crystal found in the Valley of Dead Heroes, west from the entrance to the Necropolis (M12.14). Examine it - you should receive a vision of a man plagued by nightmares. Your job is to learn the whereabouts of the woman from the vision.
Follow the main quest for now - enter the Necropolis and eliminate the whisperlock (M13.3). Details on how to do that can be found on the page describing On the Builder's Trail quest.
If you've decided to help the Children of The Endless Gate, neutralize the lock and talk to Motley. Ask him about Choi - this will lead you to crypt 5143, where Oddwald can be found.
If you've helped the Memoralists, the location of the tomb can be found in a note left by Motley - you can either find it at his corpse, or he will drop it once you chase him away from the Necropolis.
Regardless of how you've acquired the combination, approach the terminal (M13.3). You can input the code (5143) or choose Oddwald's tomb from the list of known locations.
Once you've been teleported to crypt 5143, talk to Oddwald. Use all of the dialogue options and ask him about Choi. You'll receive the access code to her tomb (4221).
Note - While you're inside the tomb, take a peek inside the interactive container located in Oddwald's tomb. Ignore his warnings and take the Electrostatic Merecaster. You can find the item in your inventory and use it to enter the Mere to see an interactive scene. However, this step is fully optional and you don't need to perform this action to proceed forward.
Use the terminal to access Choi's tomb (4221 - choose it from the list or input the combination manually). Enter the tomb and investigate the huge stone pillar found in the central part of the room. Follow the wind readings to learn that Choi's surname is Cai'Sul.
Leave the Necropolis and visit the Memoralist camp located in the western part of the Valley of Dead Heroes (M12.6). Meet with Ronos Cai'Sul and ask him about Choi. Ronos won't be able to help you, but you'll receive the code for his tomb - 2413.
Return to Necropolis and input 2413 to access Ronos' tomb. Once you've entered the room, talk with Silver and investigate the head-like object. Use all dialogue options, including investigating the archives. You'll receive the access code to Marcysa's tomb (1254)
Note - Now would be a good time to begin Endless Horror, as both quests have a very similar course, including getting inside tomb 1254.
Access Marcysa's tomb (1254). You'll encounter a small group of cultists - they will run through the open portal no matter what you'll do.
Do not follow them - first investigate the stash marked on the screenshot above and touch the symbol to pick up the Twisted Metal Merecaster. You can try to use it, but you'll be unable to start the interactive scene. You can now enter the portal to The Endless Gate (M13.4).
Complete the journey through The Endless Gate. You'll finally reach an arena (M13.6). Talk to Inifere and choose the first dialogue option to touch upon the subject of the acquired Merecaster. You can proceed in two ways. You can convince Inifere to open the Merecaster (you may need a bit of Persuasion or Intimidation to get there). If you've managed to do so, an interactive scene will play out (details below). The second method involves using Deception. Tell Inifere that you've managed to unlock the merecaster and you are able to read its contents. This will allow you to scare the man away. The quest will be completed regardless of your approach, yielding a reward of 40 Experience Points.
Freeing Choi: If during your meeting with Inifere you've convinced him to open the merecaster and enter the mere, you'll be able to free Choi during its duration. Wait for the moment when the main hero attempts to convince Inifere to release the girl. To be successful, complete a roll using Persuasion or Anamnesis. Thanks to your actions, Choi will be freed from Inifere's grasp and will join you in The Calm. You can talk to her during your next visit (simply die in any circumstances or use a different way to access The Calm). The conversation will unlock Choi's Balm, a passive ability granting +1 Training Level to Endurance.
To conclude the whole affair, return to Cai'Sul Tomb (use the code 2413 and investigate the silver monument), then visit Choi's tomb (4221), and finally meet with Ronos' impostor in the Valley of Dead Heroes (M12.6). Completing these actions will enable you to score Peace Bringer, a trophy connected to Choi's quest.