Make a room where the lights go out as soon as you first get in. Make a light cube or something that'll act as a flash light (tie a dynamic light to it) and make the player use it as a flash light through the offices of Aperture Science (corridors are always creepy when lit properly).

Don't make it anything complicated to the point where you're just having to look for where to go next with the flashlight the whole time. Just make it so that the play is scared to walk ahead at full speed and create a bunch of triggers that do random things to scare the player. Crushers that almost kill you, a cube launched by a faith plate that passes right in front of your face with no warning, etc.

The big finale could be you walking into an empty room with the lights off and then your cube shuts off without any good reason for it to. At this point the player would stop, scared shitless in their tracks. Turn the lights on and you're surrounded by 20-ish turrets (maybe less if the lag it creates is too much) and just as they're activating and maybe even right after one shot is fired, have the lights turn off and kill them from the map.

Turn the lights back on and continue regular testing.

Actually, that's very similar to the Surprise trap in my traps map except the cube is sitting in the light and as you approach it the lights go off and you heard, "Deploying Surprise in 3, 2, 1." That's when the light turn on and there are turrets in the room.

I never thought of having a flashlight cube before, Raulness you deserve a thumbs up.

Be going through a dark corridor in the BTS theme with a ton of pipes and stuff and then suddenly have a bunch of red lights flicker and cast shadows and sparks flying from different parts of the map, and then the lights all turn back off after a second or so.

You should make it co-op and single player too! You know so if I play it I won't be to scared to continue like with gm-ghosthunt2 on Garry's Mod.

Perhaps I can make 2 different versions of the map. One for SP and the other for Co-op. If I do make 2 different versions, I'll release them both together.