If you want to become the new ruler of the lands of Dementia in Oblivion Remastered, then the Ritual of Dementia is the only way.

Oblivion Remastered’s Shivering Isles DLC presents players with the opportunity to become a Duke/Duchess. You primarily have the option to rule one of two lands, the first one being Mania and the other Dementia. The latter is a land of mystery and gothic-inspired culture.
To obtain the highest seat in Dementia, you must speak to a charming fellow named Sheogorath and complete the Ritual of Dementia quest in Elder Scrolls IV. This action will affect the story later on. So, it is no ordinary side quest. Here’s the complete walkthrough of it.
How To Complete the Ritual of Dementia Quest in Oblivion Remastered

The Ritual of Dementia quest in Oblivion Remastered is part of the Ritual of Accession storyline. In it, players can pick between two regimes to rule in the Shivering Isles DLC by disposing of their active leaders.
To become the Duke of Dementia, you must speak to Sheogorath during the Ritual of Accession quest. While conversing with him, you can choose to kill the Syl, Duchess of Dementia, and take her place. This action of yours will trigger the Ritual of Dementia quest.
You will then have to go and speak to Arctus. He will ask you to speak to two individuals named Anya Herrick and Kithlan instead. They are the servants of the Duchess and can tell you quite a lot about her daily life. Which, in turn, can present you with the perfect opportunity to eliminate her. However, the servants are quite reserved, so before you ask them about Syl, you must raise their disposition to 70 or higher.
Uncover the Secret Cave

Anya and Kithlan will inform you that the Duchess of Dementia is currently in hiding. But you can still find her with the help of her two now traitorous servants. After that, Kthland will hand over the keys of House Dementia to you, and Anya Herrick will distract two royal guards so you can move safely.
Now, you simply must go to the Private Garden of Syl. There, defeat or move past the two guards stealthily if you can. Then, go to the ruler of Dementia’s private quarters. Surprisingly, you will find the Duchess dead on her bed. However, you will quickly learn that it is a ruse for her enemies, and she has flown to a secret passage.
To open the passage, you go to the northeast area of the Private Garden. Once there, interact with the statue, and the new path will open. This newly opened area will lead you to Xirethard, a hidden cave. Be careful inside the cavern because it is swarming with various traps and Dark Seducer guards.
Become the New Ruler of Dementia

Keep following your quest marker while safely navigating through the area, and at the end of the cave, you will find Syl. She will be expecting you, and a fight will break out. There is no alternative way to interact with her. So, defeat her in direct combat and take her heart.
After that, go back to Sacellum Arden-Sul. Sheogorath will be there alongside an Altar. Place the slain Duchess’s heart on the Altar, and a cutscene will begin. Once it ends, speak to Sheogorath again to complete the Ritual of Dementia quest and become the new Duke/Duchess of Dementia in Oblivion Remastered.
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