TL;DR: A dragon (Green) creates a set of Headbands of Intellect with a catch: the owners of these items must do whatever she asks or lose their (19 int) smarts. Creatures of 'low' intelligence (less than 8 int score) are the easiest targets and long-term victims.
1/ The Grand Green Lady herself / green dragon
2/ The Coil of Brilliance / magic item
3/ Typical Targets to Turn Tricky (and not want to go back to being 'stupid') / NPC set
1/ The Origin Story: The Great Green Lady
A very long time ago a slain Great & Very Olde Mommy Dragon left behind a horde for her children. Actually, the horde was already partially looted (these heroes and adventurers were making several trips… hey, gold is heavy) and the quick and clever dragonlings took whatever they could. Turns out the adventurer guarding the loot wasn't that clever after all. The smallest of the baby green dragons gets an apparently useless Tome of Clear Thought (already used / blank). No one knew this at that time but the writing in these tomes comes back every 100 years, giving the reader two (2) intelligence points each time this happens. True, for most shorter-lived creatures this is a one-shot book - but this really works amazingly well for dragons (and liches… but that is another story which shall be told at another time).
Down through the centuries this dragon ends up being rather smart. Terrifyingly so, actually.
Long before she is even Ancient, she has more intelligence than anything, including most illithid brain pools, aboleth mind-gatherings or even the gods of knowledge. She realizes that knowledge is power, sure - but being able to control who gets knowledge is ABSOLUTE power. With the combination of her intelligence, magical ability and the capacities of her servants she makes the Coil of Brilliance:
2/ Eternal Coil of Brilliance
This device gives 19 intelligence to whomever is connected & attuned to part of it, identical to the Headband of Intellect. This silver rune-encrusted coil can break off at any point, giving 'parts' that can re-attach if so desired. The detached loop changes shape enough that it can be worn as a ring, headband, nose ring, toe ring, belt, ankle bracelet, necklace, cloak pin or virtually anywhere else a loop (of any size you like) can fit. After one hour ('short rest') one's attunement kicks in and the intelligence of the user rises to supra-genius levels - and does not vanish unless the attunement is broken.
Curse: This item contains sentience and can speak to anyone who is connected to a part of this Coil. It requires any user attuned to this device to obey - lest the attunement is severed and the vast intelligence vanish.
3/ A Motley Crew of NPCs
The DMG R.A.W. 'Headband of Intellect' does not seem to require a minimum intelligence. This means that The Great Green Lady has a vast repertoire of otherwise-dim creatures under her power:
Still, The Greatworm Smoogg does do the Green Lady's bidding and has been more than foundational for setting up a vast and wide network of tunnels, resource logistics, planning and, of course, spying.
Please steal this idea and let me know if it was any fun at all. If you are inspired, i am thankful beyond words. Letting me know your story is all the thanks i would ever need.