Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 3 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Horde. The horde can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Medium creature. The horde can't regain hit points or gain temporary hit points. Once the creature is reduced to half hit points it is reduced to large size. Once the horde is reduced to hp it is replaced with 1d4-1 Zombies with 1 hp and no Undead Fortitude trait.

Multiattack. The horde makes one slam attack against each creature that occupies its space.

Slam (Horde has more than half hp). Melee Weapon Attack: +5 to hit, reach 0 ft., one target. Hit: 11 (2d10 + 1) bludgeoning damage. If the target is in the center of the Zombie Horde this attack is rolled with advantage.

Slam (Horde has less than half hp). Melee Weapon Attack: +5 to hit, reach 0 ft., one target. Hit: 6 (1d10 + 1) bludgeoning damage.