Undead Fortitude. If the zombie swarm takes more than 15 points of damage at once, it can make a Constitution saving throw with the DC being 5 + the amount of damage taken. On a success, it halves the damage.

Swarm. The zombie swarm can occupy another creature's space and vice versa, and can move through any opening large enough for a Medium zombie. It also can't regain hit points or gain temporary hit points.

Slam. Melee Weapon Attack: +6 to hit, reach 0 ft., any targets inside the swarm's space. Hit: 18 (4d6 + 4) bludgeoning damage, or 9 (2d6+2) bludgeoning damage if the swarm has half of its hit points or fewer.