That's an interesting idea. I have a witch enemy concept in the works however over the time her concept evolved to a more of a demonologist type of mage who has familiars like imps or subjects like captured villagers turned bimbos, and uses magic to enhance them in various ways. This enemy however will be more fitting in a witch coven dungeon than a forest. There will be a corrupted spellcaster fairy introduced later in the game who will be in the forest area. Here's how the Night Fairy looks like.
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When it comes to the origins of the potions the fairies use, there is an explanation already for it as the succubi forces work behind the scenes. Be it either enslaving others, forging alliances or infiltrating towns. By the time the player meets fairies the succubi have already struck some deals with the Fairy Enclave.