Deep Dark Carnivorous Moths

Carnivorous moths inhabit the shadowy depths of the Overworld. They are attracted to light and will accumulate on slow-moving players in light level 7 or higher. They not actual entities, they appear as particles flying in your face and will not hurt you right away. The longer you stand still the more moths you will see swarming around you. After ~30 seconds of combined time, enough moths accumulate that you start taking damage. To shake them off you have to jump or sprint, which will make vibrations. And as long as you are standing in light, they can start accumulating on you all over again.

Moths make it tricky to camp in the Deep Dark because you have to manage time spent sneaking quietly and time shaking off moths, potentially drawing attention to yourself. The moths only spawn in the pockets of Deep Dark and will leave when you exit the biome so they won't be a nuisance anywhere else.

I chose light as their trigger because it makes lighting a liability in the darkest biome. It makes players use it strategically, such as using torches sparingly, dim candles, or night vision potions, so the Deep Dark stays dark. Another idea is that they are attracted to food, so they will accumulate on idle players with full hunger.