The Mind of Madness

Walkthrough
While walking through the streets of Solitude, the Dragonborn meets Dervenin, an apparently slightly crazy old man who explains that his master has gone on vacation and left him. He asks the Dragonborn to get his master so that the two can be reunited and hands over the Pelagius' Hip Bone, saying it is necessary to enter the Pelagius Wing of the Blue Palace.

Gain access to the Pelagius Wing of the Blue Palace
While the hip bone may be necessary, the Pelagius Wing Key is better, so the Dragonborn must either get the key from Falk Firebeard, or by tricking one of the maids, Una or Erdi. Falk will not give up the key unless the Dragonborn has already performed some service for him.

Use the key to gain access to the Pelagius Wing
Once the key has been obtained, the Dragonborn can then enter the Pelagius Wing which is a state of severe neglect.

Search for Dervenin's master in the Pelagius Wing
Searching the wing will result in being teleported to another realm where Sheogorath, the Deadric Prince of Madness, is having tea with his friend, Pelagius the Mad.

Use the Wabbajack to escape from Pelagius's Mind
To convince Sheogorath to leave, the Dragonborn must agree to "treat" the mind of the mad king by using Wabbajack in three different scenarios: helping a young Pelagius with his night terrors, improving his self-esteem and dealing with his overbearing mother.

Escape from the Blue Palace
Once accomplished, Sheogorath relents and after some pontificating, Dervenin appears and finally the daedric prince spirits the Dragonborn back to the Blue Palace.

Trivia

 * The Wabbajack that is given to you during the quest that you must use to escape is in all actuality a different item as it has unlimited charges and the description reads "It's all you've got".

Der Geist des Wahnsinns