ffraeth

Well, that could be seen as an inversion of the loathly lady/sovereignty goddess tropes, where a man fated to be king has sex with an ugly crone only to find out she's a beautiful, otherworldly woman who confirms his kingship (e.g. Echtra Mac nEchach/The Adventures of the Sons of Eochaid Mugmedón, featuring prehistoric Irish king Niall Noígíallach/Niall of the Nine Hostages, but the tropes are found -- together or separately -- in many other tales).