Horde. The horde can occupy another creature's space and vice versa. The horde can't regain hit points or gain temporary hit points. Single-target spells and effects like Hold Person have no effect. The Horde breaks up to five Zombies after it is reduced below 1/3rds hit points.

Target Rich Environment. Zombie horde takes double damage from area damage which target at least half of the space it is occupying.

Undead Nature. The horde is made of undead that don't require air, food, drink, or sleep.

Multiattack. The Zombie horde does three Slam attacks while over 2/3rds of its hit points and two when over 1/3rds.

Slam. Melee Weapon Attack: +7 to hit, reach 0 ft., one creature in the horde's space. Hit: 8 (2d6+2) bludgeoning damage.