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.
Actions
Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.
Condition immunities: poisoned. Senses: darkvision 60 ft., passive Perception 8 .. Languages: understands the languages it knew in life but can't speak. Saving throws: Wis +0. Speed: 20 ft..
RollQuest ships the D&D 2014 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.