Eu não posso responder isso sem estragar os episódios atuais.
Nimue is the very first dark one. As a result, she is the only one who can truly defeat the darkness within each new dark one. In order for the darkness to be defeated, Nimue needs to either be destroyed, or overpowered so that her darkness cannot snuff out the light. Either way, it's basically up to Nimue to stop it.