One of the turning points in the Shivering Isles expansion of Elder Scrolls IV: Oblivion is choosing to side with either Mania or Dementia. There are two sides of madness, but you're only one person and lord Sheogorath allows you to take over one of these sides with honor, murder, and class.
No side is better than the other. In fact, they are both quite literally evil in all of the Daedra madgod's glory. But choosing a side offers rewards and story outcomes of a slightly different nature. And it involves a lot of deaths.
When You Have To Start Choosing Sides
As you do Sheogorath's bidding in Oblivion, there will come a time where you must go to a place called Cylarne and light something called the great torch. Before you do this you must deal with the conflict of the Golden Saints and Dark Seducers there. They are at each other's throat as to who should control Cylarne. Instead of mediating the fight and making sure both make it out unscathed, you must choose who you want to fight with. Kill the Golden Saints with the Dark Seducers or kill the Dark Seducers with the Golden Saints.
How To Become The Duke Of Mania
After you light the flames at Cylarne and choose whether the Mania or Dementia altar gets lit with it back in Bliss and Crucible, it's time to become a duke. You're able to listen to what both mad priests have to say about the great flames and how to actually go about becoming duke or duchess. For Mania, you must poison Thaden's food.
Find Thaden's secondary and ask her for help in killing their lord. It's tradition after all. Wide-Eye can be found either in the Halcyon Conservatory or in Bliss. Bribe her enough so that she will tell you the daily routines of Thaden. Speaking to Wide-Eye reveals there's a secret silo filled with Bliss' favorite drug. You must follow her and sneak inside. This place will be a cave full of Golden Saints you can either sneak past or murder in this quest.
When you get to your destination there will be a mound of Greenmote and Wide-Eye. After you collect the Greenmote your quest will update and say you need to sneak inside another restricted area to effectively poison Thaden's meal before it gets to him. The quest explains that you must wait for the right time of day to sneak in and drug the food. Press T on the keyboard to wait and change the times of day.
Follow the quest marker around New Sheoth's palace grounds until you find the door to Mania's private quarters. You can lockpick the door and sneak through the halls filled with Golden Saints. You can use any item that has a status effect called Life Detection to see when there are guards around the corners. Go left two times and then down some stairs to find the prepared meals. There's a Thadon wine bottle on the shelf full of bottles and his prepared food on the table.
Go back to Mania's main throne room where there is a large dining table. Use the wait time to wait until everyone is at the table and ready to feast. Thaden will eat, make a toast, and die. Interact with his body then go back to the church of Arden-Sul. Interact with the bowl on the altar to complete the ritual.
How To Become The Duke Of Dementia
Becoming the ruler of half of an evil realm ain't easy. You will have to either sneak past guards or strike them down. This isn't Cyrodiil anymore, killing is permitted as the madgod see's fit. Take the Duchess of Dementia's heart and burn it to become the duchess yourself.
To become ruler of Dementia you must sneak into Syl's bedroom and murder her. But it isn't that easy. Talk to both of her secondaries Anya Herrick and Kithlan, who always sit by her throne during the day. They will offer to help if you bribe and persuade them, allowing you to enter the Private Gardens with only minimal fighting of Dark Seducers instead of a lot.
Once you enter the Private Gardens directly behind the throne chair, there will be two Dark Seducers who will see you instantly. Killing these guards won't trigger problems anywhere else in the palace. After doing this, going to her bedroom reveals she's dead already. Or not. Wait around for Kithlan to show up and explain that Syl has pulled a fast one on you and that you must find her secret escape.
The way to enter the dungeons she's escaped through is by leaving her bedroom and interacting with the statue bust in the wall directly to your left. As soon as you enter there will be fireballs shooting from every wall. Hug the wall to avoid them and press the buttons to deactivate them. From here, there will be a series of Dark Seducer fights throughout the hallways until you find Syl herself armed and ready. So be prepared to fight your way through a dungeon.
Once you defeat all the Dark Seducers by Syl's side and Syl herself, she will drop a decent weapon and her very own heart. Loot her heart off of her body and take it back to the church of Arden-Sul in town. Placing it in the bowl upon the altar in the middle of the room will complete the quest to become ruler of Dementia.
The Outcome Of Becoming The Duke Or Duchess
Sadly or not, the outcome for both endings is the same. If you become the duke or duchess of Dementia, Thaden storms the church with objection and disobedience to Sheogorath. Because of this, Sheogorath banishes Thaden. But not before Thaden declares he will join the Order, claiming it's better than being part of a kingdom with no order at all.
If you become duke or duchess of Mania, Syl will storm the church with the same objections which complement her character more than it does Thaden. Syl will announce she's joining the Order against you all and like in both endings, Sheogorath allows Syl to leave the palace grounds alive.
If you rule Dementia you will have the power to summon Dark Seducers and the spell Blessing Of Dementia. Vice versa, you would have the ability to summon Golden Saints and have a spell called Blessing Of Mania. You'll get a Ring Of Lordship for this quest, which...