Online:Castle of the Worm (quest)

Quick Walkthrough
# Go to The Harborage and talk to The Prophet.
# Enter the portal.
# Talk to Lyris.
# Find a way into the Castle.
# Enter the Inner Keep.
# Reach the top of the Tower of Bones.
# Destroy the warded door.
# Reach the Tower Summit.
# Talk to Abnur Tharn.
# Defeat the undead minions.
# Flee the Tower Summit and escape Coldharbour.
# Return to the Harborage and discuss your next moves.

Detailed Walkthrough
After you enter a large city, the Prophet will speak to you.
:"The fog has lifted, Vestige. The path before us has been revealed. It is both dark and twisted. Come to the Harborage. We have an unexpected guest who demands your presence."

In the Harborage, speak to the Prophet. The guest is a projection of Abnur Tharn. Tharn knows Sai Sahan's location, but he will not tell the Prophet. He demands to speak to you.

Next, talk to Abnur Tharn.
:"Mannimarco and his vile master have dark and terrible plans for this world. If you wish to stop them, you must find the Amulet of Kings. But to find the Amulet, you must first locate Sai Sahan."
:And you know where he is?
:''"If Mannimarco learns that we've spoken, he'll have me executed. I'm a danger to him. I know too much. Therefore, I, Abnur Tharn, Chancellor of the Elder Council, Overlord of Nibenay, formally request asylum."
:Will you come to us?
:"Would that I could … My proposal is this: if you retrieve me from Mannimarco's fortress and grant me asylum, I will give you Sai Sahan's location in return."''

The Prophet tells Lyris Titanborn to go with you and opens a portal to Coldharbour.

The Wretched Squalor
Talk to Tharn after traveling through the portal. You are in the Wretched Squalor, a soul shriven shanty town outside Mannimarco's castle. However, Tharn has no ideas on how to get you and Lyris into the castle.

Talk to Lyris. She suggests finding a soul shriven guide and stealthily taking out the guards. 

Exit the house, and speak to one of the slaves outside. They suggest Cadwell, who was just chased into a building near the waterway. 

Cadwell is guarded by Warden Cornexius. Defeat the warden, then talk to Cadwell. He suggests you enter through the cistern. He meets you just inside, past the door to the Castle Cistern. He asks you to keep enemies away from him while he opens the doors.

He tampers with three steam vents, then the doors open and you talk to him again. Tharn is at the top of the Tower of Bones. 

Make your way through the inner chamber to the tower.

There is a conversation between two Worm Cultists going on immediately within the inner chamber.
:Worm Cult Savage: "No, no, hear me out. How many Tharns does it take to rule an Empire?"
:Worm Cult Assassin: ''"Would you shut up? I'm trying to work here."
:Worm Cult Savage: "None. The one on the throne is a patsy and the one in the tower isn't useful anymore!"
:Worm Cult Assassin: "That's not even funny. Get back to work, would you?"
:Worm Cult Savage: "What? It's true. Lord Mannimarco is going to peel his skin off and make a particularly ugly flesh atronach out of him, mark my words."
:Worm Cult Assassin: "And if you don't help me, I'll make sure you're sewn up right alongside him. Now shut up!"

thumb|right|Construct your atronach
Building a Flesh Atronach
Talk to the projection of Abnur Tharn. The door at the top of the tower is magically sealed and will turn anyone who touches it into ash. Tharn thinks you could try creating a flesh atronach and smash the door with it.  

First, you need four "chunks of flesh", otherwise known as  . These are scattered around the area and can be looted from the corpses of flesh atronachs. There are enough parts to be had from Flesh Atronachs and on the floor of the ritual chamber that fighting the boss-level opponent to gather the two chunks next to it is not necessary and it can be treated as an optional encounter.

Next, go to the laboratory and use the ritual circle. Then, Tharn directs you to read from the ritual book. Use the Ritual of Atronach Binding, and a Runty Flesh Atronach will crawl out of the pile of flesh.

:Abnur Tharn: "Ha! We'll make a necromancer out of you yet."

The atronach walks to the door, smashes it, and collapses into a pile of goo.

Freeing Tharn
Tharn is guarded by two Worm Cultists. Defeat them and talk to him.

:"Mannimarco isn't doing any of this to serve Molag Bal. His ultimate plan is to out-scheme the God of Schemes…and to replace him…With the Amulet at his command, Mannimarco believes he can trap Molag Bal's essence and absorb his power. He seeks to become a god himself!"''

He is interrupted by Mannimarco's return.

:Mannimarco: ''"Now, now, Tharn. No telling my secrets. Go to your room, Abnur. I will deal with you shortly. Ah, dear Lyris. Your old position at the Foundry is still available. We'll even give you your own set of unbreakable chains. And you. The thorn in my side. The pebble in my shoe. How strong are you? Shall we test your limits?"

He summons three waves of enemies, then Tharn reappears. When Mannimarco appears, a barrier will surround the central area. Players crossing the barrier will die and the barrier will not drop until Mannimarco is defeated. 

Abnur Tharn: "Mannimarco! You dare attack the chancellor of the Elder Council?"

He distracts Mannimarco, so you can defeat the King of Worms. However, he is only momentarily defeated. Talk to Tharn. He suggests escaping through another exit than the one you came in. He and Lyris talk as you lead the way.

:Lyris: "You must be the ugliest princess I've ever rescued from a tower, Tharn."
:Tharn: "I'm shocked that your lack of education and repeated blows to the head haven't dulled your wit, Titanborn."''

At the end of the western path is a door. There's no way out. However, Cadwell runs up from the other side and unlocks the door for you. You talk to him and suggest he come with you to Tamriel. You can choose to bring him back or not. Open the Path of the Scorned and drop into a pool of water and bodies.  Swim out and follow Cadwell to the Prophet's portal.

A Trick, A Brawl and the Truth
Once you return to the Harborage, talk to Tharn. 

:'''My estates, my titles, my own life—it's all forfeit now. All in the name of preserving the Empire. I swear by all the gods, Mannimarco will pay dearly for this."
:Are you all right?
:"Of course. I am Abnur Tharn. It will take more than a few brainless cultists to rattle these old bones."
:You were going to tell us where Sai Sahan is.
:"Ah. Yes. Ahem. Truth be told, I don't know the precise location, but I do know that he is Mannimarco's prisoner, and that the King of Worms is trying to pry the location of the Amulet from him."
:You don't know where he is?
:"No, now, before you lose your temper, let me clarify. I don't know precisely where he is, but I can find out. I'll just need a little time."
:You tricked us.

At this point, Lyris punches Tharn.

The Truth Revealed
:The Prophet: "Enough! Abnur had no choice, Lyris. And we are guilty of our own deceit, are we not? The time has come for the Vestige to learn the truth!"
:Abnur Tharn: "What's this? The Vestige doesn't know? Oh, this is rich!"
:The Prophet: "Vestige. Until now, you have only known me as the Prophet. But that is not my true name. I…am Varen Aquilarios!"
:Abnur Tharn: "Our fallen Emperor, in the tattered robes of a Moth Priest, blind as a newborn kitten!"''

Talk to the Prophet. He was afraid you would kill him if he told you the truth. He offers to let you kill him after you stop the Planemeld, if you still judge him harshly. He certainly feels responsible for Molag Bal's invasion of Nirn. 

You need to find Sai Sahan, but Varen will confer with Tharn first. He will contact you again when he has more information.
 

Notes
Prior to the One Tamriel update, this quest could not be started until level 20.
 

Quest Stages