You explore a 3D maze, attempting to kill off the phantoms in the maze. You get an audible clue when phantoms are nearby, rapid heavy breathing. Your gun has a two second recharge time, and there is also a teleport in the maze which takes you back to your start.
You can have either modern or retro graphics.
This game is now nearly thirty years old, but the original was notoriously creepy. I think I've managed to get some of the original 'creepiness' in the game.
This game is free as in really free, it is open source, there are no adverts, nothing you can buy, sell or rent, no extra levels. It's just there for a bit of fun if you want. Or not if you don't.