Soot

Sleeps inside houses of random people and let's be honest, can be a real bitch to clean. Good luck if you want to get rid of Soot!

Well, Soot doesn't have much positive to be said about it, it mostly tends to form in chimneys connected to fireplaces, for a while it's pretty harmless but a large deposit collected inside a chimney can ignite and create a chimney fire. Hot stuff, but also pretty dangerous.

So even though it's generally useless and even harmful (long-term exposure to air pollution containing Soot increases the risk of disease), still we can't deny Soot is really hot, in fact Soot is theorized to be second-largest cause of global warming.

Needless to say nobody wants Soot, but almost everyone who has to keep their village houses warm has to deal with it sooner or later. But oh well, everyone's trash is always someone's treasure, perhaps there is some great use for Soot that is yet to be discovered...