Description:
Known as the Mad God, Sheogorath is not so much a symptom but the very cause of madness. Contemporary sources indicate that his roots are in Aldmeri creation stories; therein, he is 'born' when Lorkhan's divine spark is removed. One crucial myth calls him the 'Sithis-shaped hole' of the world.
History:
Those that have played Arena may remember being sent to a dungeon by the Mages Guild of Lilandril on Summerset Isle - a quest to the Temple Of The Mad God. Sheogorath did however not make an appearance in the main quest of Arena himself.
In Daggerfall, Sheogorath was the only Daedra prince to be directly involved in the main quest. The emperor's champion would find him in the Mantellan Crux, which is such a weird place that one is led to believe it is actually part of Sheogorath' plane. He could also be summoned on his summoning day, although most will probably have summoned him by accident when performing another prince's summoning ritual in a stormy night.
In Morrowind, Sheogorath played a role as a part of the dunmer temple's "House of the four corners of trouble". One of the temple quests involved the renewing of a pact supposedly made between Sheogorath and the Tribunal by Sotha Sil to keep the Daedra out of Tamriel. Additionally, the player could talk to the mad prince through his statue in Vivec, getting quested with performing a truely insane act - killing a giant netch with a small fork.
According to the lore of the Dunmer people, Sheogorath was also attempting to destroy the city of Vivec by tricking the moon Baar Dau into crashing into the city. This plan was however foiled by Vivec who stopped the moon shortly before impact and suspended it in mid-air. A rock can still be seen floating over the high fane, however it is not known with certainty whether or not Sheogorath indeed had anything to do with it.
For more information on Sheogorath, visit The Imperial Library.