So, I literally just started using steams creation kit and I wanna make a power that reduces the overall light level in the area. How do I do this? I'm guessing I need to make some kind of custom effect but I have no idea how.

From what I know, and that is not much, the cell lighting is set by the cell properties for each cell. But, people make Night Eye mods so there has to be a way to manipulate overall light levels. Perhaps looking at a Night Eye mod, but make it so things are darker instead of brighter might be the easier way to do this.

If your doing this just for cells in your mod, then look at one of the mods that have light switches and see how they do things. Not all light sources can be removed from a cell, otherwise the screen would go black, not just dark, so there has to be mechanism they tap into to make this happen.

You may also want to peak at this thread

http://www.gamesas.com/topic/1502280-mod-for-turning-out-lights/