A Strange Door is on a small island in the Niben Bay on which large stone heads contain a portal to The Fringe. The door appears after The Elder Scrolls IV: Shivering Isles has been successfully installed. Within 24 hours, the Hero will receive a message and the quest "A Door in Niben Bay."
The island is clearly visible from Bravil and the surrounding shores. It can be easily accessed by traveling east from Bay Roan Stables. Gaius Prentus, a Bravil town guard, and S'fara, a mumbling Khajiit, stand by the portal to The Fringe.
The island has a Dementia side and a Mania side, evidenced by the color and makeup of its terrain. A ramp leads up the Mania side and has Mushroom Tree Sapling, Worm's Head, Red Kelp, and Hydnum Azure growing along its path to the door.
A second path, marked with the signs of Dementia, is accessible from the bay underneath the Mania ramp. Along this path to the door, Putrid Gigantea, Fungus Stalk, Blister Pod, and a Mushroom Tree Sapling can be found growing. These plants are the only characteristic of the Realm of Madness in Cyrodiil.