How do you get back to Oblivion from Shivering Isles?

Answer 1

Obviously, first you need the Shivering Isles expansion. Go to the small island on Nibenay Bay, the one with A huge stone sculpture of three heads one to each side and the middle one with its mouth open, revealing a purple light and flying glowingpurple balls floating out of it. Also, there are bright trees and bright orange and yellow strange plants growing on one side, and a more dank looking other side with dark colours but basically the same plants. These reperesent Mania and Dementia. Try to open the door (In the middle mans mouth), if it opens answer the mans last question as "I'll do it" (Obviously) and if it doesn't open, try killing the Khajiit and knocking out the guard and it should open.

Answer 2

You return to the fringe and there will be a door to cyrodill

Answer 3

simply go back the way you came, through the portal