Undead Fortitude. If damage reduces zombie rat king to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Undead Nature. Zombie rat king doesn’t require air, food, drink, or sleep.

Turn Resistance. Zombie rat king has advantage on saving throws against any effect that turns undead.

Rotten Aura. The Zombie Rat King emanates an aura of decay in a 10-foot radius. At the start of each of the Zombie Rat King’s turns, each creature within the aura must succeed on a DC 12 Constitution saving throw or take 1d6 necrotic damage. A creature that starts its turn within the aura can make the saving throw at the start of its turn, taking half damage on a successful save. The aura extends 10 feet from the Zombie Rat King, and its effects persist until the start of the Zombie Rat King’s next turn.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

Putrid Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d6) piercing damage + 3 (1d6) poison damage.

Fetid Swarm. When the Zombie Rat King takes damage from a melee attack, it can use its reaction to momentarily fall apart into a swarm of undead rats, scattering across the battlefield. The creature that dealt the damage must succeed on a DC 12 Dexterity saving throw or take 1d4 necrotic damage as the rats bite and claw at them. After the attack, the Zombie Rat King can move up to 10 feet without provoking opportunity attacks, reconstituting itself in a new location.