: Make a new save now if you want to get both Duke of Dementia and Duke Mania in one playthrough.
Speak to Sheogorath and chose to become the Duke of Dementia. He will instruct you to kill Syl and take her heart. Speak with Anya Herrick and Kithlan who work closely with the Duchess. They will reveal some secrets about Syl and how you can reach her. You'll now need to follow Syl through a network of tunnels. When you find her, kill her and search her body. Take the heart and return to the Sacellum Arden-Sul to complete the ritual. You'll now become the Duke of Dementia.
Refer to Ritual of Dementia
for more information.