How to script a flashback/jump scare. Need it to flash up for a second or two then return to normal but can't for the life of me figure out just how to do it. Any help would be brilliant and you will be credited in final release of the map.

Need more details. UI or model in 3D space.

What makes a jump scare scary?

Right so I'll describe it so it will be the first door which I'll use for a trigger for sound and prefab model to quickly trigger for two seconds the room which is full of candles and Satanic bits. Need to quickly flash a model prefab of someone hanging from the roof then returning to the room minus the model prefab. Hope this makes sense I can set it up to show if that helps was wondering if it was possible before I put the idea into action if you get me so I'm not wasting my time. Sorry if it's not that clear or helpful but hopefully I've got across the idea.

You will probably have to stamp the prefab and select every single individual part and move them.

ent = GetEnt("modelID","targetname");
ent.origin = (X,Y,Z); --Note that XYZ is wrong. I think it's XZY
ent.angles = (X,Y,Z;
--OR
ent MoveTo(X,Y,Z); -- Animates the entities movement
wait(2)
ent Delete(); -- or maybe move it back again.

You can use similar code to rotate or to make lights get brighter or darker.

As to my answer of what makes a jump scare scary.
It's the music or audio effects. The most important thing is a really loud ear shattering scream. (Although, you don't want to deafen headphone users)

If you know someone who's really jumpy try it on them. Maybe your mother. See if your effects worked out or not.