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.

Unusual Nature.

The zombie doesn't require air, food, drink, or sleep.

Viral Aura.

Any creature that starts its turn within 10 feet of the plague spreader must make a DC 12 Constitution saving throw. On a failed save, the creature is poisoned and can't regain hit points until the end of its next turn. On a successful save, the creature is immune to this plague spreader's Viral Aura for 24 hours.

Actions

Multiattack.

The plague spreader makes two Slam attacks.

Slam.

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage plus 9 (2d8) necrotic damage.

Virulent Miasma (1/Day).

The plague spreader releases toxic gas in a 30-foot-radius sphere centered on itself. Each creature in that area must make a DC 12 Constitution saving throw, taking 14 (4d6) poison damage on a failed save, or half as much damage on a successful one. A Humanoid reduced to 0 hit points by this damage dies and rises as a zombie (see its stat block in the Monster Manual) 1 minute later. The zombie acts immediately after the plague spreader in the initiative count.

Zombie Plague Spreader