A possible monster: The knowledge eater
This monster feeds on knowledge in its environment. They move from places of knowledge to the next, leaving behind sheer emptiness and dullness.
They take in the knowledge themselves and draw their nourishment from it. They are stupid creatures on an animal level themselves. But some can gain intelligence from the knowledge they have devoured. These are the most dangerous of their kind.
In the fight against our heroes, this creature will use its claws and sharp teeth. It prefers to attack the most intelligent member of a group. But it does not just inflict flesh wounds, but draws 1d4 intelligence from a hero with each hit and heals himself many times of this amount.
So our group eventually becomes a drooling bunch of idiots. The wizard becomes useless but luckily the barbarian does not care much about the knowledge that has been stolen from him. But be careful if a hero is set to 0 intelligence, he becomes a useless heap of flesh, which no longer even knows how to move his muscles.
Our heroes get their lost knowledge back when they consume the remains of the Knowledge Eater or when a strong restoration spell is cast.
I wish you a lot of fun with the inevitable chaos that this creature will bring into any group.