Today, I present frost mites, tiny arthropods that leech heat out directly out of their prey.
Normally, frost mites are barely noticeable pests; a hand covered with half a dozen mites will feel uncomfortably cold, but hardly any danger.
But in early autumn and late spring they swarm, gathering in masses of thousands underneath rocks. A foraging animal — or adventurer looking for an entrance to a cave — that disturbs a swarm will find him- or herself covered in masses of blue specks within seconds.
Importantly, you can’t just fire an arrow at a mass of frost mites and expect to stop them. More worryingly, you can’t knock them off-balance; they cover their victim.
As such, frost mites are immune to all damage types except fire, bludgeoning, and magical damage, and they resist bludgeoning damage (taking half damage). They cannot be knocked prone.
Watch your players try to deal with that!