Location: Near the Yellow Road, northeast of Leyawiin (link goes to the Daedric Shrines Map at UESP Wiki)
Minimum Level: 10
Reward: Skeleton Key
Objective: retrieve the Eye of Nocturnal from Tidewater Cave
One of the easier shrines to find, actually. Alves Uvenim in the Leyawiin Mages Guild will mark it on your map for you if you ask, but it will probably not be necessary. If you travel along the Yellow Road (east bank of the Niben going north from Leyawiin), you'll run right across it as it's just a very short way east of the road (near the bend in the road that's about midway between the "C" and the "L" of "County Leyawiin" on your overland map). And to make it even better, Nocturnal doesn't even want an offering; just someone of the appropriate level.
Nocturnal seems to have a problem with thieves. The Eye of Nocturnal was stolen by a couple of thieves in Leyawiin and your task is to get it back. The first thing to do is find out who took it and then find out where they stashed it. If you go asking around Leyawiin about the "Eye of Nocturnal", you'll just come up empty-handed. No one knows anything about it. But just about any male Imperial can tell you about Weebam-Na and Bejeen if you ask about "Leyawiin". It seems the two have been bragging about some "valuable jewel" they have.
If you go confront the two directly, they'll deny any knowledge of it. But your journal suggests a little eavesdropping. So leave, stealth and come back in. You'll be able to overhear a discussion about whether the jewel will be safe at Tidewater Cave (which will then be marked on your map unless you already stumbled across it in your explorations). You'll get a bit of a heads-up on what's guarding the Eye if you listen through the whole conversation. The cave itself is no great shakes with a few critter encounters and very little else. For this quest, though, there are a couple of Trolls outside the cave and three more inside. These Trolls are leveled, but not too difficult to take out. Take the left passage and go left again where it Ts. It's at the end of the underwater passage.
Take the Eye back to the shrine to get the Skeleton Key. It's an unbreakable lockpick and boosts your Security skill by 40 points. It's not quite a "must-have" item, but it's almost to that level. Note, though, that you don't get the skill-level perks. The pins will move more slowly and will be much easier to lock, but when you fail to lock a pin, the appropriate number of locked pins will still drop. The only difference is that the pick won't break when they do.