The Shivering Isles, the first full Oblivion expansion will add a whole new land to explore. The Isles, which are also known as the Realm of Madness, will be divided into a northern area (Mania) and a Southern area (Dementia). A town called Split on the Mania-Dementia border has two of every citizen, and one of the quests will be to solve this doppelganger mystery.

The Shivering Isles are reachable through a door set in a strange statue and located on a small island that appears in the water in Niben Bay near Bravil. Players will be able to move between Cyrodiil–the realm of the original Oblivion–and the Shivering Isles freely.

The expansion will contain more than 30 hours of new gameplay with branching quest storylines, and the landscape itself will change according to the decisions players make during their adventures. There will also be new creatures, dungeons, and quests. Two of the new monsters include a Gatekeeper, a golem-style creature made up of stitched together body parts, and the Knights of the Order, which are made from tree roots.