Bartholomew the Rat King

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

Pack Tactics. Bartholomew has advantage on an attack roll against a creature if at least one of the rat's allies is within 5 feet of the creature and the ally isn't incapacitated.

Rat King. Bartholomew has full mental control over the 12 other rats from the bag of rats.

Multiattack. Bartholomew makes one claw attack and one bite attack

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 4 (1d8 + 5) slashing damage.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 4 (1d8 + 5) piercing damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw. On a failed save, the target takes 3d6 poison damage and is poisoned until the end of his next turn. On a successful save, the target only takes half damage.

Rat swarm. Bartholomew commands the rats from the bag of rats to form into one swarm of rats (using the swarm of cranium rats stat block). He can only do this if all 12 rats from the bag are within 10 feet of each other.

Save the King. If one of the 12 rats are within 5 feet of Bartholomew when he is hit with an attack, he can use his reaction to have one other rats take the damage instead.