At first glance, Somnia appears to be a moody first-person game set in a dark and twisted world. You can walk around its levels, jumping onto and over various platforms. And you'll be thinking, "OK, this is kinda unsettling, but where's the game?"
The game lies in the shadows.
Hidden inside Somnia is a whole new game that uses the shadows cast by its 3D world to delineate an alternate 2D environment. It's only by moving simultaneously through the 2D and 3D versions of each area that you'll be able to complete the game.
Somnia is still a work-in-progress so the developer continues to fine-tune gameplay elements. But you can download the current demo right now. I suggest you should.
Somnia [Cryptic Sea]