Morgan Le Fey and Glinda/Wicked Witch mashed potato do seem like the most likely candidates based on lands/people referenced over the last two seasons.
My own personal headcanon is that Charming’s father was a knight of the round table and left to marry Ruth. But before he died, he taught David how to sword fight and the chivalric code. Going off that, my first guess is Morgan. (and if it is, everybody should have to read the Mists of Avalon by Marion Zimmer Bradley. Because I said so)
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"He was a lot of things to me" "The only conclusion was love"