Swarm of Keese (5e Creature)
From D&D Wiki
Swarm of Keese
Medium swarm of Tiny fiends, neutral evil
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 fiend. The swarm can't regain hit points or gain temporary hit points.
Bites. Melee Weapon Attack: +2 to hit, reach 0 ft., one target. Hit: 5 (2d4) piercing damage, or 2 (1d4) piercing damage if the swarm has half of its hit points or fewer.
Clustered swarms of keese patrol the night, acting together, as if controlled by a hive mind.