UAC Military Nightmare (updated version)

The UAC is once again, concocting more groundbreaking experiments on teleportation. They thought they were ready to handle everything that came their way. While traveling between dimensions, something unexpected happened. An unknown enemy force appeared out of nowhere. An evil, much more sinister than the last, single-handedly wiped out the first UAC Marine squadron, and is gradually killing off the remaining Marines. Everyone who has seen this evil, has been annihilated. Except, for one. You. You must take on this evil all alone. As you watch the rest of you're comrades get slaughtered by this unworldly evil, you must make your way to the sinister mastermind behind all of this destruction. Can you repel the evil before you? Load up you're weapons.