Tech Overview

This game combines a few techniques to achieve the darkness affect.

2D space in this game has "light levels" sampled at every pixel.
There is a grid of "blobs" which grow and shrink in response to "light levels". 
The blob layer overlays most of the player layer, so the floor and entities become covered by darkness.

The player attacks can "reset" blobs to zero, forcing them to regrow if light level permits. Likewise, high light levels cause blobs to shrink and become 0.

Enemies pathfind, with different weights for light level and blob density. Skeletons prefer to stay in dark areas, spiders avoid light but will fight it, moths are drawn towards light...