Containment Breach is the result of several prototypes of SCP-related horror games. It's the most horrifying game I'm aware of, and I've been looking at
a lot. Keep in mind that the game becomes a lot more intense if you're actually into horror fiction and went through some of the backstory regarding the SCP facilities and their monsters. These stories are sometimes a little bit too detailed, and I love how they freak me out in a very masochistic way.
If you like SCPCB you should have a look at it's predecessors. The first SCP based game was
SCP-087. Also refered to as 'staircase simulator'. You walk down a staircase and a monster randomly appears. No big deal, this still caused heart attacks just by the sheer effect of suspension, which is pretty noteworthy.
This was later improved upon and became SCP-087-B. Same thing, just now the staircase is a maze and has more monsters added to it. Again, this game has no goal other than to die, which is also inevitable. Many people ALT-F4'd out of the game when they hit a dead end on the lower floors, refusing to turn around.
Just looking at this picture causes stress to me. Even more so when you're frantically staring into the black, relying on twitch reaction to not die when something actually comes out of there.