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

Pack Tactics. The rat 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.

Undead Fortitude. If damage reduces the zombie 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.

Multiattack. The Zombie Giant Rat Pack Leader makes two attacks: one with its bite and one with its claws.

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

Claws. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 6 (2d4 + 1) slashing damage.