Hungry Rats. Any creature that is Medium or smaller and ends its turn inside the rat king's space must succeed on a DC 16 Dexterity saving throw or else become grappled by the rat king (escape DC 16) and trapped in the rat king's body of hungry rats until the grapple ends. While trapped, the target is blinded and restrained, has total cover from attacks and other effects outside the rat king, moves with the rat king, and takes 18 (4d8) piercing damage and 7 (2d6) necrotic damage at the start of each of the rat king's turns. Creatures that are immune to disease are immune to this necrotic damage. If the rat king dies, creatures inside it are no longer trapped by it.

Many Heads. The rat king has advantage on Wisdom (Perception) checks and on saving throws against being blinded, deafened, stunned, and knocked unconscious.

Reactive Rats. When the rat king uses its reaction to make an opportunity attack, it regains its reaction at the end of that turn.

Scatter. When the rat king is reduced to 0 hit points, it dies and the remaining living rats are freed; 3d4 giant rats appear in the rat king's space or in unoccupied spaces within 5 feet of it.

Stench. Any creature that starts its turn within 15 feet of the rat king must succeed on a DC 16 Constitution saving throw or be poisoned until the start of the creature's next turn. On a successful saving throw, the creature is immune to the rat king's stench for 24 hours.

Wakeful. While the rat king sleeps, at least one of its heads is always awake.

Bestial Subtype. This creature is bestial, meaning the spells animal messenger, beast bond, beast sense, locate animals or plants, and speak with animals affect the creature as if it were a beast.

Multiattack. The rat king makes five bite attacks, but it can't make more than three attacks against the same target. If it hits a creature that is Medium or smaller with two of these attacks, the creature must succeed on a DC 16 Strength saving throw or else be grappled by the rat king (escape DC 16), pulled into the rat king's space, and trapped inside its body of hungry rats until the grapple ends.

Bites. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 10 (3d4 + 3) piercing damage plus 3 (1d6) necrotic damage. Targets that are immune to disease are immune to this necrotic damage.

Hungry Charge (Recharge 5–6). The rat king moves up to its speed and makes an attack against each creature of its choice during the movement. A creature that is hit by one of these attacks must succeed on a DC 16 Dexterity saving throw or else be grappled by the rat king (escape DC 16), pulled into the rat king's space, and trapped inside its body of hungry rats until the grapple ends.

Detach. When the rat king is subjected to psychic or slashing damage, it creates one giant rat for every 7 points of psychic or slashing damage dealt (before resistances) in unoccupied spaces within 5 feet of it (minimum one giant rat). These giant rats are allied with the rat king.