According to an investigation done by Yngvar the Wanderer, Cylarne is the oldest structure that can be found in the Shivering Isles. Yngvar dates Cylarne to be approximately thousands of years older than any other ruin.
The Dark Seducers hold the left side of the ruin and the Golden Saints the right side. There is one chest outside in the garden along with multiple urns and chests inside, all with various minor loot such as potions or coins. A key is required to enter and can be obtained by speaking to one of the guards during part of the main quest.
Cylarne - UnderkeepEdit[?]
Cylarne - Altar of DespairEdit[?]
After completing both the "Addiction" and "The Lady of Paranoia" quests, the Hero must speak to Sheogorath. He will reveal that the real reason they are in the Shivering Isles is to stop the Greymarch by becoming the new Sheogorath.