One can only defeat so many villains that want to "cover the world in darkness" before one starts to look for a different flavor of horrible.
EVIL JUST GOT EMO!
During a storm the PCs come upon a little boy that plays in the rain. Upon further inspection they discover that he has one eye and a long tongue lolling in its mouth. Spit dripping. It is a demon from beyond that can access our realm during storms, it eats all heads that are not its own, living and dead.