Jyggalag is a daedric prince whose sphere is order and uniform thought.
He had grown in power so much so that the other Daedric Princes had grown fearful and jealous of him and cursed him to live as Sheogorath the Daedric Prince of the thing he hated most: madness. At the end of every era an event known as the ‘Greymarch’ would take place allowing him to return to his true form in order to retake the Shivering Isles from his mad alter-ego but once this was done Jyggalag transformed back into Sheogorath starting the cycle again.
At the end of the third era during the dawn of the Greymarch the Champion of Cyrodill was able to defeat Jyggalag before he concurred the Shivering Isles, breaking the cycle and allowing him to remain in his true form. The Champion of Cyrodill then became the new Sheogorath, taking on his personality and likeness. Jyggalag has since made no attempt to retake the Shivering Isles and has not made an appearance anywhere well into the fourth era.
He commands daedra known as the Knights of Order, which are spawned from obelisks summoned by his followers the Priests of Order. His realm was the Shivering Isles but it was lost to Sheogorath and no attempt has been made to reclaim it. Only one artifact is associated with Jyggalag:
Sword of Jyggalag: Can damage opponents not harmed by normal weapons.
The daedric prince’s enemy is Sheogorath.
In Oblivion the player must defeat him to break the cycle and stop the Greymarch.