In the Once Upon a Time episode The Bear and the Bow, our heroes use the magic mushroom to summon Merlin. But instead of Merlin, they get Merlin's "voicemail," that tells our heroes that the only way the Dark One can be defeated is by Nimue. Why would Merlin say such a thing? Especially since we find out in the next episode, Nimue, that Nimue
was Merlin's girlfriend that became the very first Dark One.