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.

Shambling Horde. If the Zombie has at least 4 other Zombies within 30 feet, all humanoids within 30 feet of the zombie must make a DC 8 Wisdom Saving Throw at the start of their turn, becoming Frightened on a failure. If a creature succeeds on this saving throw, they are immune to its effects for 24 hours. The DC for this saving throw increases by 1 for every 3 additional Zombies above 4. 

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

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

If this attack reduces a creature to 0 Hit Points, it must make a DC 8 Constitution Saving Throw. If it fails, it turns into a Zombie in 1d4 hours. This saving throw is made with disadvantage if the creature was under the Frightened when it was reduced to 0 Hit Points.

Dousing the wound in Holy Water, casting healing magic such as Cure Wounds, or restorative magic such as Lesser Restoration all end this effect. 

Rot Toss. Ranged Weapon Attack: +3 to hit, reach 20/60 ft., one target. Hit: 8 (2d6 + 1) poison damage. If this attack hits, the target must succeed on a DC 8 Constitution Saving Throw or be Poisoned for 1 minute. A target poisoned this way can repeat the saving throw at the end of each of their turns, ending the effect on a success.