Instructions
Select the "Ritual of Mania" as your active quest. Follow the marker to Wide-Eye in Thadon's court.
Talk to Wide-Eye and ask about Thadon's dinner routine. She tells you about his Greenmote use. Ask her about her daily plans. You may need to bribe her before she talks about her plans. When she does, she mentions that she has an errand to do every day at noon.
Wait on the plaza outside Sheogorath's palace. At noon, Wide-Eye exits the building. Walk behind her and follow her to the Greenmote silo.
Sneak through the caverns in the Greenmote silo. If Wide-Eye or one of the guards sees you, they throw you out of the silo. Go to the back of the silo and take two drops of Greenmote. Exit the silo.
Enter Thadon's court at 6 p.m. Sneak into the kitchen. Wait for the guards to pass by and run to the food counter.
Add a dose of Greenmote to the bowl on the counter. Add another dose to the bottle of wine on the shelf.
Go to Thadon's dining room at 8 p.m. Wait for him to eat the poisoned meal. After a few seconds, he dies. Touch Thadon's corpse to get the blood needed to perform the Ritual of Mania.
Enter the temple in New Sheoth. When you touch the altar, you complete the Ritual of Mania. You are now the Duke of Mania.