Blood Frenzy. The swarm has advantage on melee attack rolls against any creature that doesn't have all its hit points.
Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny quipper. The swarm can't regain hit points or gain temporary hit points.
Water Breathing. The swarm can breathe only underwater.
Bites. Melee Weapon Attack: +5 to hit, reach 0 ft., one creature in the swarm's space. Hit:14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer.
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…