Method Two: Premade Mask
For this one, make a huge black image, and use a small soft eraser to cut a hole in the centre. Save it as a PNG, this will be your mask.
Now bring that image into Construct 2 as a Sprite.
In Events, set the position of the sprite:
every tick -> set mask position to character.x, character.y
You can make whatever kind of shadow you want, if you want fog, you can make a grey mask. You could create a torch beam that pierces the darkness, and make the light whatever colour you wish.
But if you want to make a beam of light, you have to add an additional Action to your Event:
every tick -> set mask angle to character.angle
Then:
Boom! We're in Silent Hill!
Now this is great and simple for large areas, but when you reach the edge of your layout you see the edge of the mask. There's probably something you could do about that, but it'll get fiddly and by that point you may as well use a different method