Swarm. The swarm can move into and stay in another creature's space, and other creatures can enter its space. It can pass through any opening wide enough for one Tiny rat. The swarm can't regain Hit Points and can't gain Temporary Hit Points.
Actions
Bites. Hit: 5 (2d4) Piercing damage, or only 2 (1d4) Piercing damage if the swarm is Bloodied.