How about a module in the Dimensions of Dream?

[JiCi]

Maybe I got into Psychonauts a bit too much recently, but I could see an adventure inside a victim's consciousness/dream and having to fight a nightmare creature as a final encounter.

[Rysky]

The House on Hook Street might not be exactly what you're asking for but it does deal a lot with dreams and nightmares.

[captain yesterday]

Yes! Highly recommended, my favorite module.

And you actually do go to the dream plane.

[Rysky]

Sweet! I've just skimmed over it since I actually want to play it so I don't know what's all in there.

Glad I wasn't that far off though :3

[Just your average clone]

My GM just ran HOHS, and it is AWESOME. (although, we did set the house on fire, thus ruining a lot of clues.)

[Niv Cel'on]

I would like more modules that go into the different planes