A definite DLC buy!!!
The story begins with the player finding a mysterious door on a small island off the coast of the town of Bravil. When you enter the player discovers that he/she is in the realm of Sheograth, which is divided into tow halves. One half is called Mania. Mania is very bright and it seems like a dreamland, besides the monsters. Dementia, the opposite half, on the other hand is darker and more mysterious.
This is a must buy download for Oblivion because it lets you have the full experience of the game. The only negative thing I can think of about the Shivering Isles download is not enough new monster and music, and repeating environment and scenery.