I'm currently writing a mod called http://www.gamesas.com/bgsforums/index.php?showtopic=829807 and I'm just trying to find anything I can about him. I know he made the skeleton key, but thats it.

Moromaster.

P.S. Does he have any rivals?

OK, referring to Nocturnal as a 'he' is probably not a good start. Granted that Daedra can manifest as whatever gender they wish, but Nocturnal is always described as female.

More helpfully, look http://www.imperial-library.info/book_daedra/index1.shtml#nocturnal.

Nocturnal is the myth-echo of the Great Void beyond even Oblivion.

Even the other Daedric Princes do not trifle with Nocturnal.

___TWM

But is there someone who REALLY doesn't like?,

There has not been anything said definitively about a formal rivalry between Nocturnal and any other entity to my knowledge. Sorry.

___TWM

This is generally true. However, two of Dagon's lieutenants did cross through part of Nocturnal's Realm during the events of Battlespire and left rather a mess.

They were probably lost though.

No. Passing through Nocturnal's Realm was part of the plan. Getting into a huge fight with Nocturnal's own daedric servants (and with each other!) on the way was not.

EDIT: http://www.imperial-library.info/tsobs/part04.shtml contains the relevant bit of the Battlespire storyline.

I was joking, but thanks anyway. I've always felt that Battlespire has some of the most excellant dialogue of all TES material ever made.

Why would you decide beforehand that your quest will be about Nocturnal when you don't know the first thing about Nocturnal?