One from Many. Every 15 total hit points the Horde has, represents one individual zombie. At 30 or less hitpoints, the Horde becomes 2 individual Zombies as the Horde is no longer a group.

Deathly Stench. Any creature that starts its turn within 10 feet of the zombie must succeed on a Constitution saving throw. (DC 10) If within 5 ft, (DC15) If surrounded by the horde, (DC 18) If Entombed, or take 10 (2d10) poison damage and be poisoned until the start of the creature’s next turn.

Unusual Nature. The zombie doesn’t require air, food, drink, or sleep.

Many as One. This creature can walk single file, enabling it to enter spaces any Medium sized creature can. 

Horde Lunge. Melee Weapon Attack: The Horde collectively swarms an area immediately adjacent to itself within 20 feet. Any creatures or objects in the area must make a Dexterity saving throw or be enveloped in the Horde. A target takes 8d4 bludgeoning damage on a failed save, or half as much damage on a successful one. 

Entomb (Recharge 5–6). The Horde converges upon an object or creature within 10 feet of itself. The target must succeed on a DC 18 Strength saving throw or take 20 (4d10) bludgeoning damage, and if the target is a Large or smaller creature, it becomes entombed in the Horde.

A creature entombed in the Horde is restrained, has total cover against attacks and other effects outside the Horde, and takes 12 (4d6) slashing damage at the start of each of its turns while Entombed.