The Shivering Isles is the second full expansion released for The Elder Scrolls IV: Oblivion, the first being Knights of the Nine. The expansion takes place in the titular Isles, which is the realm ruled over by the Madgod Sheogorath. The realm reflects Sheogorath's madness, being split down the middle between the bright and beautiful Mania, and the dark and blighted Dementia. Sheogorath watches over his lands from his palace in the capital city of New Sheoth, which like the rest of the Isles is divided into the dark Crucible and light Bliss. Like his insanity, the Palace teeters on the edge of both sides.
The player is recruited by Sheogorath to stop an apocalyptic event called the Graymarch. The Daedric Prince of Order, Jyggalag, invades the Realm of Madness at the end of every era to wipe it clean, forcing Sheogorath to rebuild his domain from scratch. This time, though, Sheogorath plans to fight back, and needs the help of the player to do it.
Unlike Knights of the Nine, which takes place within Cyrodiil, The Shivering Isles takes place on a separate map from Oblivion - the two are connected by a door in Niben Bay - and is roughly one fourth the size of the original game. The new quests provide about 30 hours of new gameplay. The expansion also includes:
- 32 new ingredients for alchemists
- 10 new creatures to combat
- 9,500 lines of new dialog
- 200 new weapons
- 80 pieces of armor and clothing