Multiattack

The rat king makes four bite attacks.

Bite

Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage, and a bitten creature must succeed on a DC 15 Constitution saving throw or be infected with a disease. A diseased creature gains one level of exhaustion immediately. When the creature finishes a long rest, it must repeat the saving throw. On a failure, the creature gains another level of exhaustion. On a success, the disease does not progress. The creature recovers from the disease if its saving throw succeeds after two consecutive long rests or if it receives a lesser restoration spell or comparable magic. The creature then recovers from one level of exhaustion after each long rest.

Summon Swarm (1/Day)

The rat king summons three swarms of rats. The swarms appear immediately within 60 feet of the rat king. They can appear in spaces occupied by other creatures. The swarms act as allies of the rat king. They remain for 1 hour or until the rat king dies.