Unusual Nature. The swarm doesn’t require air, food, drink, or sleep.

Multiattack. The swarm makes one Undead Mass attack and one Flesh Entomb (if available)

Undead Mass. Melee Weapon Attack: +4 to hit, reach 0 ft., one target in the swarm’s space. Hit: 5 (1d6 + 2) bludgeoning damage, or 4 (1d4 + 2) bludgeoning damage if the swarm has half of its hit points or fewer.

Flesh Entomb (Recharge 6). The zombie flings a detached clump of rats at a creature it can see within 30 feet of it. The target must succeed on a DC 12 Strength saving throw or take 7 (2d6) bludgeoning damage, and if the target is a Large or smaller creature, it becomes entombed in dead flesh.

A creature entombed in the dead flesh is restrained, has total cover against attacks and other effects outside the dead flesh, and takes 2(1d4) necrotic damage at the start of each of its turns. The creature can be freed if the dead flesh is destroyed. The dead flesh is a Large object with AC 8, 5 hit points, and immunity to poison and psychic damage.