Board Thread:Show-Related Questions and Answers/@comment-5817180-20151113073602/@comment-8345449-20151126053538

Perhaps the Nimue in Emma's head isn't the 'real' Nimue. We know that head Rumple isn't real Rumple, so head Nimue could just be a manifestation of the darkness using Nimue's memories and personality.

The real Nimue is likely dead and in the underworld as with all the Dark One vessels before her. So maybe Merlin is telling them to contact her somehow (using the magic ale perhaps) to seek answers.