Undead Horde Fortitude. If the horde takes damage, equal to the max HP of one of your zombies in a single attack, then the zombie horde must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit, in which case it becomes an automatic fail. On a fail, then zombie horde loses a zombie and it will lose more zombies if the damage taken is two times, three times, or four times the max HP of one of your zombies respectively.

If the horde takes damage that would put its HP lower than the number of zombies in the horde, then the horde must make a Consitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit, in which case it becomes an automatic fail. On a fail, the horde keeps zombies in the horde equal to the HP remaining. On a success, the horde keeps HP equal to the zombies in the horde.

Undead Horde. The horde can occupy another creature's space and vice versa, and the horde can move through any opening large enough for a Medium zombie. The horde can't regain hit points or gain temporary hit points.

Undead Horde Growth/Shrinking. A caster who can cast the Animate Dead spell may cast Animate Dead to increase how many zombies are in the Zombie Horde, provided the caster has a corpse to do so. This increases the horde's HP and Max HP based on the newly incorporated zombie's HP and Max HP. The horde's Max HP will always be the zombies' Max HP multiplied by however many zombies are in the horde. When the Zombie Horde loses a zombie it drops a corpse.

Undead Horde Vulnerability. The Zombie Horde is vulnerable to AOE damage.

Slam. Melee Weapon Attack: +([Zombies in Horde] / 2) rounded down a minimum of 3; to hit, reach 5 ft., one target. Hit: [Zombies in Horde] * 2 ([Zombies in Horde]d4 + [Zombies in Horde]); bludgeoning damage.