SCP-087 is a first person horror game where all you do is walk down stairs. Or, to be fair, you decide to walk down stairs. As you get further down, you may begin to hear some things that aren't very promising, or think you saw something out of the corner of your eye, it's up to you to decide to keep going or to just turn the game off.
The game constructs tension through restriction of agency, not allowing the player to do more than walk (very slowly), turn, look and jump. I tried jumping down the staircase in an effort to remind myself "This is just a game!" but it was to no avail. The game's fear is psychological and reminds us that what we fear reflects upon who we are.
SCP-087 was built in Unity and is available as a free download here.