My theory is that the monster in the cave is Maeve’s mother, who was completely consumed by her own magic.
Furthermore, I suspect that every witch lineage carries a unique, otherworldly element. For some, this manifests as an unusual hair color; for others, it leads to premature graying and stunted growth; for some, it takes the form of an inner beast that devours them from within—and for others; Someone results in insomnia.