Instructions
Travel to Leyawiin once you are level ten. Leave Leyawiin from the north gate, heading up the road along the Lower Niben River.
Find Nocturnal's Shrine to the east, located about half way up Niben Bay. Wait for nightfall to enter the shrine, as the goddess Daedra will only speak to you then.
Talk to Daedra to receive the quest to retrieve her stolen eye. Head back south to Leyawiin.
Speak to any guard in the city, bringing up the topic of the Eye of Nocturnal in the conversation. You will be informed of two individuals who have been boasting about a valuable gem and receive a new quest marker.
Follow the quest marker to Weebam-Na's house. Talk with the two and then leave. Enter stealth mode and go into the entry hallway to begin eavesdropping on them. They will talk about the jewel being in Tidewater Cave.
Follow the new quest marker to Tidewater Cave, which is on the southern point of Cyrodiil. Fight the trolls in the cave, heading left at every turn. You will eventually get to an underwater passage that takes you to the Eye.
Return to Nocturnal's Shrine with the Eye. Speak to Daedra and she will give you the Skeleton Key as a reward.