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.

Legendary Resistance (3/Day). If the monster fails a saving throw, it can choose to succeed instead.

Multiattack Zombie Santa can take two melee attacks

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: (1d8 + 5) bludgeoning damage.

Peppermint Sword. Melee Weapon Attack +6, reach 5ft., one target (2d8 +5) Bludgeoning 

Peppermint Breath (Recharge 5-6). Ranged Breath Attack 60ft, +6, two targets (5d6 + 5)

Legendary Actions (3/Turn). 

Misty Step. Cast Misty Step at will

 Peppermint Sword. Melee Weapon Attack +6, reach 5ft, one target (2d8 +5) Cold Damage

You've Been Naughty (2 Actions). Ranged attack 30 feet Santa shouts targets name the target must make a DC15 CON saving throw or be stunned and take 3d6 psychic damage, On a success the target takes half damage and isn't stunned.