Kind of gimmicky and would rely upon what materials technology is available. A board game representing a fantasy world of castles and forests, etc. with treasures and monsters. Each square has a peg hole at the corner. There is an extra game piece that fits in this hole and covers a random number of squares around it (it’s amoeba-shaped). This is ‘the darkness’ which is plaguing the land. It moves every turn. Normal squares are filled with evil when the darkness overcomes them. A field of plains suddenly becomes overrun with ghouls.
Here’s where the trick into how it works would come in: the overlay is some sort of translucent plastic where only certain inks/colors come through (preferably in an eerie blue). So, the square printed with green plains and a blue sky, when this piece is laid over it, shows a graveyard with ghouls (and specific info on what to do if you land in that square). Think along the lines of those old Transformers toys where you’d get this red strip of plastic film, hold it over a box on the back and be able to read stats on the character (used to LOVE that gimmick!). There’d be no getting around having to move characters each time the thing moved, as well as cards sitting on spots. Hmm….