The Elder Scrolls IV: Oblivion Guide - Shivering Isles
Quest 16: The Roots of Madness
Look behind the Throne of Madness, and proceed through the door marked 'The Fountainhead.' With 'The Roots of Madness' set as your active quest, start following the arrow on your compass. The Gnarl Cultivators here are tame, so do not attack them. If you accidentally kill the tame Gnarl that is initially there, activating the Gnarl Chrysalis in the area to the right will spawn another. A Gnarl Cultivator will open the Order crystal root door for you. Only the Gnarl Cultivators are tame; any other variants of Gnarl will attack you normally.
Kill the Order Priest that the arrow points you to. Then, while facing the pool, turn right and continue following the arrow on your compass. Proceed through the root door to reach the next area. To open the Root Door that is blocking your way, activate the Gnarl Chrysalis on the western side of this area to spawn another Gnarl Cultivator.
Follow the quest arrow on your compass to the Pool of Dementia. Kill all three Order Priests surrounding the crystals to cleanse the pool. If your character is the Duke of Mania, Syl will be standing on the dais here. She doesn't seem to attack, so just head up the steps and cut her down. Search the Priests' corpses afterwards, and remove the Shard of Order. Backtrack to the previous room. Do not follow your compass at this time, and instead return to The Fountainhead area the same way you entered the Pool of Dementia. Cross the pool in the middle of the room and open the Order crystal root door using the Shard of Order.
Use Gnarl Chrysalis to spawn friendly Gnarl Cultivators.
After using the Shard of Order to open the root door, follow the arrow on your compass to the door to 'The Fountainhead, Pool of Mania.' This area can be a pain to navigate if you tend to rely on your compass to point you in the right direction. Drop down to the section below and use the log ramps to cross the gap, and then follow a tunnel over to the parallel area.
Drop down and kill the Order Priests. If your character is the Duke of Dementia, Thadon will stand on the dais here. He's quite fast, but is only wearing Order Priest armour, so you should be able to cut him down quite easily. After killing Thadon, remove the Diadem of Euphoria helmet from his corpse. This helmet adds the Resist Magic effect, fortifies the Speechcraft skill, and fortifies Magicka. Follow the arrow on your compass to a door back to The Fountainhead area. Move through this area and return to New Sheoth Palace. There, activate the Font of Madness behind the throne.