There are 2 types of dark spell 1 I got from the mistress where is another one?
It's kinda out of the way. You have to beat the Mindflayer jerk as Celica after kicking his ass as Faylen.
After following the hunter lady's questline enough you unlock a dungeon in the swamp area run by a shady creepy guy. Return when possessing Faylen and use her staff skill to reveal himself as a mindflayer. Proceed to fight him (you'll want to fully level up Faylen beforehand).
After the fight you will get a ring shaped like a squid. Cna't recall if you get it automatically or you check the dungeon area afterwards.
Wear the ring in the swamps as Celecia and you'll eventually encounter a grey octopus thing. Let it take control of you until the special status effect meter maxes out. It'll bring you to the Mindflayer for a boss battle as Celecia.
My advice is to bring plenty of health, magic and anti confusion potions since it is a very tough fight. Win and he'll drop the AOE dark spell.