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 hit points or gain temporary hit points.

Foul Stench. Any living creature that begins its turn with a undead rat swarm in its space must succeed on a DC 15 Constitution save or be nauseated (poisoned) for 1 round.

Filth Fever. Any living creature that has taken damage from undead rat swarm has a d100 rolled: on a 01-35 the creature is infected with the Filth Fever. 1d3 days of incubation, damages 1d3 Dex and 1d3 Con.

Bite. 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 hit points or fewer.