Shadow Warrior

Box art

Shadow Warrior is a 1997 3D action game, developed by 3D Realms using Ken Silverman's Build Engine. It featured major technical advancements to the Build Engine (even partially surpassing Blood with it's capabilities) such as driveable armed vehicles, properly implemented room over room, and opaque water surfaces.

Due to the chaotic nature of the game, as well as unpredictable enemy placements, overpowered enemy attacks, and a great lot of 3D platforming and environmental puzzles, it can be easily considered as the hardest game from the Big Three.