Note: This is the point at which one must chose which Duke/Duchess to replace. Though one can pick either side and still complete the Main Quest, the choice will have certain consequences.
If one picks Dementia, this will start the Ritual of Dementia.
After speaking with Sheogorath, talk to both priests. They can explain Arden Sul's background. Both sides claim Arden-Sul was their duke, though their stories of his final day are wildly different.
If one chose to take the throne of Lady Syl, speak to a resident of Crucible such as Gloorolros. Once they increase their disposition, they will tell them that Syl is rarely seen in public and relies on her aides Kithlan and Anya Herrick.
Access to House of DementiaEdit
Speak to Kithlan first and raise his disposition enough and he will give them a key that opens any door in the House of Dementia. If one raises Anya's disposition enough, she will also help by distracting two of the guards. Let her enter the Private Gardens first. They will have to fight two Dark Seducers before one can enter Syl's private chambers. There her dead body is on her bed. However, Kithlan jogs up and tells them that the dead body is merely a decoy, and they must follow her into a secret passage in the Gardens.
The secret door is in the Gardens, next to Syl's private quarters, hidden behind a bust of Sheogorath. Activate it and it will slide back to reveal a passage down to Xirethard, an ancient ruin.
It is a heavily trapped ruin. When inside watch out for a network of passageways guarded by drain-health-spitting statues. The map will be invaluable for navigating this section. A switch at the end will turn off the statues, allowing time to explore the halls, where potions and gold lie.
Fight past some Dark Seducers and into Xirethard Depths. There will be two Seducers in a circular room. Kill them and look for a switch on the north side of the room, at the base of a bust.
Follow the hallway around until one get to a long room with a staircase on the south side. A single Seducer will be guarding the top of the stairs. Behind her, a doorway leads to another room. Syl is in this room and, if the innumerable traps and guards weren't an indication, will be difficult to take down. No longer wearing her regal gown, Syl is now clad in Seducer armour and packing real heat in the form of Nerveshatter. Kill her and take her heart. Don't forget to look for a chest with leveled loot in the room.
On the east side of the great room is door to Xirethard, Subterrane. This level is filled with undead. Fight them all the way through, taking care to avoid statues that shoot spells and collapsing ceilings.
Once outside of Xirethard, go back to Sacellum Arden-Sul. Put Syl's Heart on the altar. It will glow for a few seconds and then disappear into a bluish-green burst.
Arctus, High Priest of Dementia, will declare the Hero Duke/Duchess of Dementia. Speak to Sheogorath and he will congratulate them on their new title. Suddenly, Thadon bursts into the temple, devastated by Syl's death and worried about the Greymarch. He says he will leave Sheogorath's side and serve Jyggalag as a Priest of Order. Thadon also says that the Fringe has been taken over by the forces of Order.
Speak to Sheogorath again and he says that he is pleased by Thadon's treachery, as this has never happened before, and new is good. He will give the Hero the Ring of Lordship. He also thinks that Thadon is correct about the Fringe being lost and sends them to investigate.
At this point, they also have full use of the Duchess' Private Quarters. Take anything one wants, sleep in the bed and store anything in any of the containers. Every container in the Quarters is safe and will not reset.
This ends your current quest and begins the next: Retaking the Fringe.
To learn more about Syl's routine, I should begin by asking around New Sheoth about Syl to determine the best way to get close enough to kill her.
I've removed Syl's heart. I now need to make my way back to the Altar of Arden-Sul at the Sacellum.
After touching the Altar of Arden-Sul, there was a brief flash of light. Arctus, the High Priest of Dementia, has now proclaimed me Duke of Dementia.
After informing Sheogorath of my victory, Thadon, the Duke of Mania became furious. Before he departed the Sacellum, he threatened that we hadn't heard the last of him. I should speak with Sheogorath about the next step in his plan.
Sheogorath has awarded me the Ring of Lordship. I am now officially Duke of Dementia of the Shivering Isles.