Undead Fortitude. If damage reduces the zombie swarm 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 swarm drops to 1 hit point instead.

Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Small humanoid. The swarm can’t regain hit points or gain temporary hit points.

Slam. Melee Weapon Attack: +4 to hit, reach 0 ft., one target in the swarm’s space. Hit: 12 (2d8 + 3) bludgeoning damage, or 7 (1d8 + 3) bludgeoning damage if the swarm has half of its hit points or fewer.