The Citadels of the Sixth House

Summary

 * Quest Giver: Starts Automatically after entering the Ghostgate
 * Prerequisite Quest: Hortator and Nerevarine
 * Required Rank: None
 * Next Quest: None
 * Location:Ghosttown, Dagoth Ur
 * Characters Involved: Dagoth Ur
 * Creatures: Hunger, Bonewalker, Dremora, Golden Saints, Corprus, Clannfear, Ash Slave, Dreamer, Ash Zombie, Ash Vampire, Daedroth, Cliff Racer, Rat, Kagouti, Guar, Dagoth Priests, Skeleton
 * Reward: Enchanted ring, Keening, Sunder
 * Disposition Increase: + 10 to almost everyone in Vvardenfell
 * Fame Increase: + 20 reputation

Quick walkthrough

 * 1) Head through Ghostgate
 * 2) Head to either the fort with Keening in or the Fort with Sunder in
 * 3) Kill the Ash Vampire in the fort to get the weapon
 * 4) Get the other weapon from the other ash Vampire in the Other fort
 * 5) Head to Dagoth Ur (Levitation Magic recommended but not needed)
 * 6) Keep heading down into the fort battling enemies along the way
 * 7) Speak to Dagoth Ur
 * 8) Once you've finished "you can strike the first blow"
 * 9) Kill Dagoth Ur and pick up the ring that appears
 * 10) Walk through the Door and run to the right (avoiding Dagoth Ur) and keep heading down.
 * 11) Equip Sunder and cross the bridge to the heart.
 * 12) Hit the Heart with Sunder ONCE
 * 13) Equip Keening, while ignoring Dagoth Ur's attacks and keep hitting the heart till it dies.
 * 14) Now attack the Mortal Dagoth Ur till he dies
 * 15) Run away from the Statue and watch it fall in to the lava.
 * 16) Head back through the door you went through (if its locked there's a crank nearby that unlocks it)
 * 17) Speak to Azura and watch the cutscene
 * 18) Take the ring and leave
 * 19) You have Saved Vvardenfell and possibly all of Tamriel, the weather around red mountain has changed and all of Vvardenfell knows your name.

Detailed walkthrough
It is easy to get lost around red mountain, one good idea is to use levitation magic to get to all the forts or cross the lava to Dagoth Ur.