Horde. As  a swarm type creature this creature gains benefits accordingly. As a whole this creature has (4) 1d4 hp per zombie in it and has an AC of 8. It can fit through spaces 5x5 without squeezing, additionally it can occupy the same space as another creature. If the number of zombies exceeds 20 the horde size is considered Huge.

The hoard starts with Multiattack when it has 5 zombies. For every additional 5 zombies it gains an additional attack. If the horde is not occupying the same space as another creature it has only half of its attacks rounded up against it.

Damage: For every 5 zombies after the initial 5 the zombies do an extra 2d4 on a hit.

Optional: The horde gains an extra +1 to hit for every additional 5 zombies

Multiattack: starting with 2 (5 zombies) and gaining an additional per 5 zombies (ex 15 zombies have 4 attack)

Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) bludgeoning damage.