Oblivion - How to access the Shivering Isles expansion content

The Shivering Isles expansion for Oblivion sends you to the Fringe to discover the secrets within and can offer some pretty good loot and is available to you right after exiting the sewers at the start of the game.

Upon exiting the sewer tutorial area, a notification will alert you to a strange door being spotted in the nearby Niben Bay, located just outside of Bravil. When you wish to begin the Shivering Isles, fast travel to Bravil and swim to the strange island in the middle of the lake.

You will find an odd structure composed of large stone faces and a glowing gateway there. Speaking to the nearby guard will trigger another nearby individual to begin rambling and attack you.

After dealing with the attacker, tell the guard that you wish to go in.

A few moments later, you should hear a mysterious voice emanating from the portal saying things about finding a mortal champion. Once you hear this, you can enter the doorway into the Fringe and begin the expansion's questline.