Morgana's Athame mentioned so prominently in the early books, connection to OG Merlin. Fit's with Arthurian Fae Lore.
Makes sense.
My first thought was that it was a stretch...but then it occurred to me that it might depend on which myth has the story right.
According to several sources, Arthur was more-or-less raped by one of his sisters using witchcraft, and this was how Mordred was conceived. (The version I heard most often is that the spell made him have a dream that he was making love to his wife, when he was actually with a witch.)
Now which sister varies. Some accounts have it being Morgan, others Morgause.
There's also a story that Nimue (in most of these versions a student and lover of Merlin) trapped Merlin (the original) in an enchanted slumber, too.
Now we know from recent DF works that Mab was 'involved' with Merlin, and is still sensitive on the subject...
The thing is that the myths conflict, but somewhere in the tangle is whatever actually happened back then in the DV. So while I doubt Mab is Morgan...I'm pretty sure she knew Morgan well. I wonder if Mab might not be Nimue, though. IIRC one of the enemy taunted Mab about being cast out by Merlin, maybe he broke out of the enchanted sleep and was in a bad mood or something.
