Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a medium humanoid. The swarm can't regain hit points or gain temporary hit points. A swarm is vulnerable to area of effect abilities.

Power in numbers. Depending on the number of zombies in the swarm the collective receives an attack for every 5 zombies in the swarm. The number also affects the size of the swarm as well as gives it new abilities the bigger the collective grows.

0-5 (Medium):
Whispers of the Dead: The swarm has a chilling ability to move silently, its movements more akin to a gentle rustling than the expected cacophony of groans and dragging feet. This eerie silence, coupled with a supernatural ability to blend into the shadows, makes the swarm much harder to detect than one might expect. In this size the zombie swarm is harder to detect, giving it an advantage on stealth checks in suitable environments. 

6-15 (Large):
Grasping Hands. As the swarm expands into its large form, the swarm can reach out with numerous hands to trip, grab, or slow down its foes. Enemies attempting to move through or escape the swarm's grasp will need to succeed on an athletics check against the number of zombies in the swarm or be grappled until the end of their turn.

16-25 (Huge):
Chaotic Onslaught: As the swarm expands into its huge form, the sheer number of zombies within its mass creates a maelstrom of frenzied attacks that overwhelms its foes through sheer chaos. This multitude of undead limbs striking from all directions makes the swarm's assault unpredictable and difficult to defend against. This chaotic onslaught grants the swarm advantage on attack rolls against any target it has surrounded within its numbers. 

26-40 (Gargantuan):
Terrifying Moan: The swarm emits a collective, mournful moan that strikes fear into the hearts of those nearby. Enemies within 30 feet need to succeed on a wisdom check of half the number of zombies or become frightened.

Collective Endurance. When the swarm suffers damage that would normally disperse individual members, it instead demonstrates a horrifying resilience unique to its collective nature. For every 10 points of damage received in a single attack, the swarm loses one of its constituent zombies but otherwise continues its advance undeterred.

Lunar Necrosis. Under the eerie glow of a blood moon, the Zombie Swarm surges with malevolent energy, each zombie pulsating with dark vitality.
- Empowered by this celestial phenomenon, the zombies' attacks become infused with additional necrotic potency. Each successful strike inflicts an additional 1d6 necrotic damage upon their victims, draining their life force with relentless hunger.
- Guided by the sinister influence of the blood moon, the zombies' movements become eerily precise. They gain a +2 bonus to hit with their strikes, their movements seemingly synchronized with the celestial alignment, ensuring their blows find their mark with chilling accuracy.

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage.