SCP-087 is a 3D horror game, based on the stories of the same name. It's a simple proposition: there's a staircase, and you can't escape by going up, so you have to go down. Into the dark.
Made in Unity, SCP-087 is a very dark and very creepy game. The game is procedurally generated generated, which means every play through will be a little different. Nothing much happens in SCP-087, but the horrific sounds and darkness make it a very scary experience. It doesn't have the shocks of Slender, but it's still a game that's difficult to keep on playing.
If you enjoyed this, you should try SCP-087-B, and SCP – Containment Breach, which explore similar themes, and are equally chilling!