Two Divisions of Shivering Isles

Earlier I learned that Shivering Isles is divided into 2, the Dementia and Mania. You cant access these places until you kill the gatekeeper which seems to be almost impossible but with a quest activated you can kill it. Dementia represents the darker side of the Shivering Isles....I'm kinda lazy right now so heres the official link for the Dementia and Mania