Fell taint

Fell taints are tiny alien predators from the Far Realm. They attack the minds of their targets, generating madness and insanity so that they can feed off their victims' psychic energy. They are animalistic things driven almost entirely by instinct, possessing little intelligence, yet are somehow able to communicate with each other and coordinate against larger prey. They can slip into the Material Plane through even the smallest cracks in reality, and in great enough numbers can even cause those cracks to widen enough for more horrifying things to come through.