Just in case you wont to know the easy way but doesnt look so good


create a sprite the size of the area you want dark.

Make sure its black and transparency is off.

Then make a object for it and put this code in the step event


image_alpha=0.5


Change the second number to change how dark it is

1.0 = no change
0.0 =completely transparent

Then put the objects where you want them.