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

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

Destroy Undead Weakness. If the zombie is fails a saving throw against a Cleric's Destroy Undead, CR 1/2 or higher, the zombie horde is destroyed. 

United Front. The swarm has advantage on saving throws against any affect that targets a single creature. As well as advantage on Strength checks and saving throws. 

Multiattack. The zombie can make 3 slam attacks. 

Slam. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 26 (6d6 + 5) bludgeoning damage, or 15 (3d6 + 5) bludgeoning damage if at half health or fewer.