This requires a minimum level of 10. (If you are not yet that level I recommend the Arena. Quick way with many rough fights to improve your skills.)
Begin by fast-traveling to Leyawiin(far south), and exit on the east side. Follow the dirt road north a ways and on your right should be the Shrine of Nocturnal, the Night Mistress. Her shrine requires no offering, only being level 10.
She speaks of her 'Eye of Nocturnal' being stolen by two argonians in Leyawiin. Check out Weebam-Na and Bejeen at their home. Talk to them about it and they seem kinda nervous responding with things like
"Never heard of the thing. Neither has Bejeen. You ask to many questions."
Your journal will alert you to try to eavesdrop. I would recommend in the corner between the bed and the wall and out of sight behind their bed curtain thing. You will hear them talking about Tidewater Cave and check your map and its south on the coast of the Topal Bay.
You will face few things, just critters and up to 5 trolls depending on your level. The 'Eye' itself is underwater glowing with several colors so it shouldnt be hard to miss. Return to the Shrine with fast travel or walk if you choose and she will reward you with the...
Skeleton Key
An unbreakable lockpick that increases your security by 40.
Good Luck!
