Light demons make rubble with their special exploding effect. When a light demon spawns it is weak, and easy to kill. If you let it be for a couple minutes so it can gain in strength then it will have the potential to explode. Basically you whack on it then wrestle with it for long enough till it starts drawing in balls of white to power up. After about half a minute it will blow a ring of fire then a larger ring of white fire, stand about ten tiles away when it does that or you'll get exploded as well. Anything in the vicinity of that ring will get exploded, some of the stuff will get moved around, and some of the highlightable tiles will turn into rubble. The rubble that was exploded will keep spawning for up to an hour, so sit in the area and wait till it stops spawning.