The Nature of Swarms

The swarms presented here aren't ordinary or benign assemblies of little creatures. They form as a result of some sinister or unwholesome influence. A vampire can summon swarms of bats and rats from the darkest corners of the night, while the very presence of a mummy lord can cause scarab beetles to boil up from the sand-filled depths of its tomb. A hag might have the power to turn swarms of ravens against her enemies, while a yuan-ti abomination might have swarms of poisonous snakes slithering in its wake. Even druids can't charm these swarms, and their aggressiveness is borderline unnatural.