Undead Fortitude. If damage reduces the zombie to 0 Hit Points, it makes a Constitution saving throw (DC 5 plus the damage taken) unless the damage is Radiant or from a Critical Hit. On a successful save, the zombie drops to 1 Hit Point instead.
Condition immunities: exhaustion, poisoned. Senses: Darkvision 60 ft.; Passive Perception 8. Languages: Understands Common plus one other language but can't speak. Speed: 20 ft..
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.