A gwyllion is a hag that, while malicious and hungry for tasty babies, will behave itself if you treat it politely.
Once your life and all your worldly possessions have been surrendered to her, a gwyllion can be surprisingly civilized. (Nip Gwyllion)
She speaks only in vile epithets, spitting out curses and obscenities that harm others just by the hearing. (Harvest Gwyllion)