SRD (p.384)
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 insect. The swarm can't regain hit points or gain temporary hit points.
Bites (swarm has more than half HP). Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: (4d4) piercing damage. Bites (swarm has half HP or less). Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: (2d4) piercing damage.