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

Undead Fortitude Horde. When Horde of zombies are grouped together add advantage to saving throw. on saving throw success add 10 hit points instead of 1.

Horde Movements. When this unit moves they move in relative unison. With sufficient intelligence the unit can even sequentially march with the skill of a skilled military unit. tight formations come with risks. If attacked with any AoE spell or attack take disadvantage on saving throws X2. the Attacker also gets advantage X2 on attack rolls if attacking with any AoE attack or spell. On critical successes of attacker or critical fails on saving throws by defender the damage is quintupled.

Horde Slam. Melee Weapon Attack:  reach 5 ft., one target. The zombie horde attacks the target with rotten fingers and broken teeth. make a dexterity or  constitution  saving throw. dc15 on fail  take (6d6 + 6) bludgeoning damage. on save take half.

Horde Grapple. The horde attempts to dog pile on the target. make a strength check +6 with advantage to a saving throw of athletics or acrobatics. on fail the target is grappled and cannot move or be moved.