Keen Smell. The rat has advantage on Wisdom (Perception) checks that rely on smell.

Speak with Rats. The Rat King can communicate with normal rats (including Giant Rats).

Multiattack. The Rat King makes three bite attacks

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (1d4 + 4) piercing damage.  Hit: If the target is a creature, it must succeed on a DC 12 Constitution saving throw or contract Bone Break Fever.  Until the disease is cured, the target can't regain hit points except by magical means, and the target's hit point maximum decreases by 3 (1d6) every 24 hours.  If the target's hit point maximum drops to 0 as a result of this disease the target dies.  The disease can be cured with a successful medicince check (DC 14) or if the creature survives 3 continuous days after the disease is contracted.