Online:Halls of Torment (quest)

Quick Walkthrough
# Go to the Harborage and talk to the Prophet.
# Choose a companion.
# Enter the portal.
# Defeat Abnur's doppelganger.
# Defeat Lyris' doppelganger.
# Defeat Varen's doppelgangers.
# Defeat the Duchess of Anguish.
# Return to the Harborage and discuss your next moves.

Detailed Walkthrough
When you approach a major city, Varen Aquilarios appears and announces that they are ready to rescue Sai Sahan.

Head to the Harborage. When you enter, Lyris Titanborn appears to be fighting with Abnur Tharn about accompanying you.
:''"Put a cork in it, Tharn! Sai's my friend. Probably the best friend I have. If the Vestige is going, I'm going too!"
:"Yes, I see. And you are the most qualified, why? Is it your superior knowledge of Coldharbour? Your expertise in dealing with Daedric antics? Or is it simple the unrequited puppy love you feel for that shirtless mammoth?"''

Talk to Varen. It took him awhile, but with Tharn's help he found the Halls of Torment. Lyris and Tharn volunteered to help you in Coldharbour, but Varen fears their mutual hatred may hinder your task. He asks you to choose one to accompany you.

Tharn offers his skills as a mage and his knowledge of the Daedra. Lyris offers her dedication and loyalty-she would give her life for you or Sai. Once you've made your choice, enter Varen's portal to the Halls of Torment.

Inside the Halls of Torment, you hear Mannimarco mock Sai and demand the location of the Amulet of Kings. As you get closer, you hear Tharn's voice mock Sai and question him. It is a doppelganger of Tharn.

:Tharn Doppelganger: ''"Becoming a sword-singer is child's play, Sai. I mastered it in an afternoon. Here, let me show you."
:Tharn Doppelganger: "Where is the Amulet of Kings? Tell me, or I'll gut you like a fish with your own sword!"

When you get close enough to the scene, the Doppelganger will attack.
:Tharn Doppelganger: "Witness the deadly power of my dark sword-singers!"

Defeat the doppelganger of Abnur Tharn. The fight is relatively straightforward. The doppelganger stays on or near the platform and casts skeletons at you. Fight the skeletons and engage the doppelganger with ranged attacks. If you get too close, his area of effect attack will kill you.
:"How touching! Your friends are here to save you. Pity they arrived too late. It's time for your next round of torture. You'll enjoy this one."'''—Duchess of Anguish

Sai is pulled into the blue flames. Talk to your companion. If you are with Lyris, she thinks the Daedra know that Tharn's jokes hurt Sai deeply. If you chose Tharn, he'll explain how they were once rivals in a forgotten martial art called sword-singing and take an interest in how insults from him affect Sai so deeply. Sai's failure to become a sword-singer was his greatest failure, so seeing Tharn, his rival, master that art was torture. You continue to the Chamber of Dark Seduction. Outside the door, you hear someone who sounds like Lyris ask for the Amulet's location in exchange for their being together forever.

:Lyris Doppelganger: "Sai, my love. Where is the Amulet? If you tell me, this will all be over and we can finally be together."
:Lyris Doppelganger: ''"Just tell me where the amulet is and we can be together forever, Sai. Isn't that what we both want?"''

Inside, Lyris' doppelganger shields herself by drawing power from four stones. If you destroy the stones, you will be able to defeat her and reach Sai. However, Seducers appear from portals whenever you get close to the stones. Once the shielding stones are down, she begins to deal with you herself. Defeat the doppelganger of Lyris Titanborn. This fight is largely the same as the Doshia fight at level 8.  Kill every orb that spawns or you will die. Otherwise you just have to wear her down. 

Defeat the Duchess|thumb|right
However, Sai is once again wrenched from your grasp. Talk to your companion. If you are with Lyris, she denies that she and Sai are anything more than good friends. If you chose Tharn, he seems to take it as a confirmation of a long held suspicion. Continue into the Chamber of Agony. Here Varen's doppelgänger asks Sai why he didn't protect him and claims he cannot trust him. However, when you attack one of the Varens, the Duchess of Anguish appears to fight you herself. Defeat the Duchess of Anguish. She is a powerful caster with an attack that will knock you down. She also summons a flame atronach to keep you occupied.

Sai is finally free. Talk to him. He is surprised that Varen is still alive, and he never gave up the Amulet's location. Enter the portal to the Harborage, and speak to Varen. After Sai has recovered, you will help retrieve the Amulet of Kings. Varen reveals that he knows the Amulet is in the ruins of Sancre Tor, but you need to speak to Sai. Sai says he left the Amulet in a sacred catacomb, protected by magical wards. Kasura, his old mentor, is the only one who has the means to lift the wards. He gives you a reward, happy to rest for now in the company of friends.

Notes
Prior to the One Tamriel update, this quest could not be started until level 30.
 

Quest Stages