Wikipedia
Nimue (Once Upon a Time)
"Nimue" is the seventh episode of the fifth season of the American fantasy drama series Once Upon a Time, which aired on November 8, 2015.
In this episode, Merlin helps Emma, while Regina, Mary Margaret, David, Hook, and Robin are tricked by Zelena. In Camelot's past, Merlin falls in love with the woman who becomes the first Dark One.