It's not Cornwall, but Hoel is a mythical king of this region, whose folktales are preserved in the Barzaz Breiz.
This region's grottoes are inhabited by red-eyed, priest-hating female spirits that seduce men at twilight near large stones. The last person to die in this region during the year takes over as the blind, cart-driving psychopomp Ankou. According to some myths from this region, a girl who strangles her lovers with a magical black mask is finally punished by being transformed into a morgen. It's not Cornwall, but Hoel is a mythical king of this region, whose folktales are preserved in the Barzaz Breiz. A kingdom in this region was founded by Conan Meriadoc, whose son (*) Gradlon fathered a girl who stole a key from a hunchback to please the monstrous Cado. That sinful enchantress, Dahut, brings about the sinking of this region's City of Ys. For 10 points, name this region in northwestern France.
This region's grottoes are inhabited by red-eyed, priest-hating female spirits that seduce men at twilight near large stones.
{Brittany} [or {Bretagne;} or {Breton} mythology; prompt on {"Celtic"} mythology; prompt on {"France"} or other less specific answers; do not accept Britain]