I think it comes down to two people Wendy or Tink who i think is Morraine. For Wendy when the shadow got Bae there was a book that opened up that was blank that look like a lot the once upon a time book. What if Wendy wrote the book for herny to understand the curse after all Wendy told Bae he did not believe like Emma. As for Tink well August said a fairly told him about the book and Rumple and his dagger I think the person was Tink. Reason why tink would know Tink could be Morraine she had a crush on Bae like Tink did Peter I think Morraine found out about neverland from the Blue fairly and she ask to be transform into a fairly so she could find Bae after all if she was a fairly she could come and go from neverland to london as she pleases. I will love it if Bae was Peter Pan and Morraine was Tinkerbell 😀