Hive Mind. The Rat King is composed of a mass of individual rats sharing a single consciousness. It has advantage on saving throws against being blinded, deafened, stunned, or incapacitated.

Swarm Form. While the Rat King has more than half its hit points, it can occupy the space of other creatures and can move through openings as small as 1 foot wide.

Splintering Flesh. When the Rat King takes bludgeoning, piercing, slashing, or force damage, 1d8 rats break free from its form and enter combat, rolling initiative separately. The Rat King cannot be reduced below half of its maximum HP by this effect.

Multiattack. The Rat King makes three attacks: one with its Bite, one with its Tangled Claws, and one with Tail Lash.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. 

Hit: 14 3d6 + 4 piercing damage.

If the target is a creature, it must succeed on a DC 15 Constitution saving throw or become Diseased. While Diseased, the creature has disadvantage on Constitution saving throws and ability checks until it completes a long rest.

Tangled Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. 

Hit: 10 2d6 + 4 slashing damage, and the target must make a DC 14 Strength saving throw or be grappled (escape DC 14). While grappled this way, the target takes 3d6 piercing damage at the start of its turn as the rats gnaw at them.

Tail Lash. Melee Weapon Attack: +4 to hit, reach 10 ft., one target.

Hit: 12 2d8 + 4 bludgeoning damage, and the target must make a DC 14 Dexterity saving throw or be knocked prone.

The Rat King can take up to three legendary actions per round, only at the end of another creature’s turn.

• Scurry (Costs 1 Action). The Rat King moves up to half its speed without provoking opportunity attacks.
• Bite Frenzy (Costs 2 Actions). The Rat King makes two Bite attacks.
• Call the Horde (Costs 3 Actions). The Rat King summons 2d4 additional Giant Rats that burst from its mass and act immediately on their own initiative.