Best forgotten game ever, I swear! Here on GOG! Buy it!
Maybe try to write a few words what make it so special, that would certainly be very helpful :)
3d puzzle/platformer with some shooting/action - much like Tomb Raider early games in general design.
However, in atmosphere and in delivery, much superior! The world of Shadowman is the dual-world of Voodoo. You are dealing with a back-and-forth between two worlds. The voice acting is marvelous. The level design is fantastic. The atmosphere is completely unique.
Bonus: there is a boss fight in a Vietnam War level which features the best Doors-imitation music ever created!
The game is fun enough to finish and then go back to play through again to try to and 100% find every item and collectible. A thing which I nev er do!