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.

Multiple Heads. The zombie has four heads. While it has more than one head, the zombie has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious.  

Multiple Arms.  The zombie has six arms.  While it has more than three heads, it has advantage on saving throws against being grappled, restrained, or knocked prone.  

Whenever the zombie takes 12 or more damage in a single turn, one of its arms falls off.  

Reactive Arms. For each arm the zombie has beyond two, it gets an extra reaction that can be used only for opportunity attacks.  Additionally, if the zombie has three or more heads, creatures, provoke opportunity attacks even if they use the Disengage action before leaving the zombie's reach.  

Multiattack.  The zombie makes as many slam attacks as it has arms.  

Slam. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage.

Flailing Parry.  The zombie adds +1 to its AC for each arm that it has (maximum of +5).