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!
In the middle of a desert in an oasis is a single prospering oak tree. Near the tree is a large pool of water. Drinking from the water is said to give you eternal life but really only extends your life for as long as you continue drinking from it. Those that have found it and left, have never found it again. Some search for it still.