Reach level 10
Start the Daedric quest for Nocturnal at the Nocturnal Shrine north of Leyawiin
Speak to the shrine and accept the quest
Travel to Leyawiin and find the stolen Eye of Nocturnal
Follow the quest marker to the Argonian thieves
Retrieve the Eye of Nocturnal
Return the Eye to the shrine
Receive the Skeleton Key as the reward