The Elder Scrolls IV: Oblivion Guide - Shivering Isles
Quest 6: Addiction
Thadon, the Duke of Mania, can be found in the House of Mania, most likely in the Halcyon Conservatory area. Have 'Addiction' set as your active quest, and follow the arrow on your compass to locate the Duke. Through Thadon's incoherent babble, you'll learn of an item called the 'Chalice of Reversal.' He can't tell you where this item is, so you'll find someone in the castle who can.
Look for Wide-Eye the Argonian in the House of Mania or in the Halcyon Conservatory. Ask her about the Chalice and she'll mention a Dunroot Burrow. Inquire about this location further; if her disposition is high enough, Wide-Eye will tell you that one can only enter Dunroot Burrow if they have consumed Felldew. This substance can be acquired from a variant of the Elytra.
Dunroot Burrow has been marked on your map; as you can see, it's quite a ways away from your current location. It can be difficult to reach this location if you try to take an indirect root. The north road leaving Bliss will take you directly to Dunroot Burrow. Travel along this road until you reach Camp Hopeful. At Camp Hopeful, follow the dirt road straight to Dunroot Burrow.
Glowing Elytra yield Felldew, which you'll need to consume to enter Dunroot Burrow.
Outside of Dunroot Burrow, look for an Elytra that will yield Felldew. An Elytra whose body is faintly glowing holds Felldew. Kill the Elytra Hatchling and remove the Felldew from its body. The Barrier Membrane at the entrance to Dunroot Burrow cannot be opened unless you have consumed some Felldew. Locate the Felldew in the Ingredients section of your character's inventory and eat it. When the membrane opens, proceed inside.
You're going to want to move through Dunroot Burrow rather fast. The Felldew effect does not last forever; when it wears off, it will put your character into a state of Felldew Withdrawal, crippling him or her to an extent. The effects may not seem too bad at first, but they worsen as the withdrawal persists. Plenty of Elytra in Dunroot Burrow should hold Felldew, so as long as you take care not to loiter, you should be able to make it through without a hitch.
Follow the arrow on your compass to the door into the 'Dunroot Burrow, Kelp Fen' area. The initial area of Dunroot Burrow contains several different levels; when you are unable to progress on the current floor, look around for a wood ramp leading up to the next level. The Kelp Fen area is comprised of many tunnels; refer to the local area map here to avoid traveling in circles.
Consume Felldew to temporarily reverse the effects of Felldew Withdrawal.
Continue following the arrow on your compass to the 'Dunroot Burrow, Drone Tunnels' area. This area is simple to navigate; just follow the arrow on your compass to reach the 'Dunroot Burrow Bramble Halls' area. Following the arrow on your compass in the Bramble Halls won't help, since the direct path to the next area is blocked. You'll have to walk all the way around. Look to the local area map if you are having trouble finding your way.
The Chalice of Reversal is in the 'Dunroot Burrow, Sanctum of Decadence' area. Once you've reached the end of the Bramble Halls and are in the Sanctum of Decadence, head up the stairs. The Felldew Addicts in this area are hostile, but they aren't armoured at all, nor do they have a weapon equipped. The Chalice of Reversal is on a pedestal in the middle of the upper area of the tower, so follow the arrow on your compass to it and take it into your inventory. There's no need to backtrack through all of Dunroot Burrow, as some stairs in this area lead up to a door back out to Sheogorath's Realm.
Sanctum of Decadence, kill the Felldew addicts there, and grab the Chalice of Reversal.
Back outside, you'll encounter an Obelisk of Order, which acts as a spawning point for the Knights of Order. To destroy an Obelisk, place three Heart of Orders inside. Doing so will cause it to overload, closing it in the process. Once an Obelisk of Order has been closed, you can usually retrieve a few items from it. Be sure to collect each and every Heart of Order from the defeated Knights. With the Chalice of Reversal in hand, return to Thadon in the House of Mania and hand it over to him.