After fighting a sectarian battle in Cylarne and then relighting the Great Torch in the Sacellum Arden-Sul, Sheogorath appears in the pews, clapping delightedly. He says that the Greymarch is rapidly approaching.
He would like the Hero, for the first time in history, to stop the Greymarch. He says that he is tired of picking up the pieces afterwards and always has trouble remembering where New Sheoth goes. He also explains that his previous effort of filling a pit with clowns (clouds?) was ineffective and aromatic.
Note: This is the point to chose a side and pick which Duke/Duchess to replace. Picking either side still will complete the Main Quest, but this choice will have certain consequences.
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.
Speak again to Sheogorath and this is the final time to chose a side. If picking Dementia, the quest Ritual of Dementia will begin. If picking Mania, the quest Ritual of Mania will begin. Either choice ends the current quest.
I should report to Sheogorath that I have lit the Great Torch as he requested.
I've spoken to Arctus, High Priest of Dementia. The Ritual of Accession for Dementia is simple: Syls would have to be slain by the proposed successor and her heart brought to the Altar of Arden-Sul at the Sacellum. I should now speak to Dervenin to learn of Mania's ritual.
I've spoken to Dervenin, High Priest of Mania. The Ritual of Accession for Mania request the current ruler to commit suicide by taking three doses of Greenmote, which is lethal. Then his drug-tainted blood must be brought to the Altar of Arden-Sul at the Sacellum. After I decide which ritual to undertake, I should return to Sheogorath and inform him of my choice.