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

Zombie Grab Swarm: When you hit a creature or get hit, you can make a grapple check against it as a bonus action.

Strength In Numbers: The Horde of Zombies makes a number of attacks and deals damage based on its size and Increases HP while becoming harder to hit due to the number of them.

While the Horde of Zombies size is Endless Swarm, when it attacks, it makes 10 attacks, and deals 8d6 + 6 damage on a hit and x 2hp +1 AC.

When the Horde of Zombies is reduced to 3/4 its maximum hit points, its size becomes Gargantuan. While the Horde of Zombies size is Gargantuan, when it attacks, it makes 5 attacks and deals 5d6 + 4 damage on a hit and x 2hp +1 AC.

When the Horde of Zombies is reduced to half its maximum hit points, its size becomes huge. While the Horde of Zombies size is huge, when it attacks, it makes 3 attacks and deals 3d6 + 2 damage on a hit and x 3hp +1 AC.

When the Horde of Zombies is reduced to a quarter of its hit point maximum, its size becomes large. While the Horde of Zombies size is large, when it attacks, it makes 2 attacks and deals 2d6 + 1 damage on a hit and x 5hp +1 AC.

Multiattack - The Horde of Zombies makes 10 attacks when its size is Endless Swarm, 5 attacks when its size is gargantuan, 3 attacks when its size is huge, and 2 attacks when its size is large.

Bite and Clawing: Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 4 (1d6 + 1) Bludgeoning damage.

Large Swarm: Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 4 (2d6 + 2) Bludgeoning damage.

Huge Swarm: Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 4 (3d6 + 3) Bludgeoning damage.

Gargantuan Swarm: Melee Weapon Attack: +6 to hit, reach 5 ft., 1 target. Hit: 4 (5d6 + 4) Bludgeoning damage.

Endless Swarm: Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 4 (8d6 + 6) Bludgeoning damage.

Zombie Grab Swarm: When you hit or get hit by a creature, you can make a grapple check against it as a bonus action.