Undead Fortitude: If damage reduces the Horde to a point that a Zombie would fall, 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 remains in the Horde.
When the Horde takes damage, all Zombies over the Horde threshold repeats a saving throw (example: At 63hp there are 5 zombies, as two continue to succeed on their saving throw. These 2 repeat the saving throw when damage is taken as the HP threshold is for 3 zombies).
If damage drops the Horde to 0hp, any successful saving throws drops it to 1 hit point instead.

Horde Movement: The Horde consist of 9x Zombies. These Zombies move as if one creature in a single grouping (3x3) or in a connected line. They move as one unit.

Horde Size: The Horde starts with 9 Zombies, each one a medium creature. For every 22hp damage taken, the Horde loses one 1x Zombie.
>176hp = 9 Zombies
155-176hp = 8 Zombies
133-154hp = 7 Zombies
111-132hp = 6 Zombies
89-110hp = 5 Zombies
67-88hp = 4 Zombies
45-66hp = 3 Zombies
23-44hp = 2 Zombies
1-22hp = 1 Zombie

Multiattack: The Horde is able to make 1 Slam attack for every Zombie that remains and is within range. Attacks can be split to target up to 3 different creatures.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

Formation Change: As a Bonus Action, the Horde can move its ranks to form a tighter cluster (filling any gaps) or forming a line.
This does not impede on movement or actions. All Zombies in the Horde must remain adjacent to one or more unless only one remains.