Rat, Swarm of Rats

Accustomed to Filth. The swarm has advantage on saves against contracting diseases.

Heightened Smell. The swarm's Perception is 15 when perceiving by smell.

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 rat. The swarm can't regain HP or gain temporary HP.

Swarm Resilience. The swarm is resistant to bludgeoning, piercing, and slashing damage, and it is immune to the charmed, frightened, grappled, paralyzed, petrified, prone, restrained, and stunned conditions.

Bites. _Melee Weapon Attack:_ +2 to hit, reach 0 ft., one target in the swarm's space. _Hit:_ 7 (2d6) piercing damage, or 3 (1d6) piercing damage if the swarm has half of its HP or fewer. If the target is a creature, it must make a DC 9 CON save. On a failure, the target contracts the rat plague disease (see Diseased Rats sidebar) or is poisoned until the end of its next turn (the GM's choice).