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.

Sense for Flesh: The undead has double movement speed and advantage on all attack rolls for the first round of combat.

Horde Tactics: The undead has advantage on attack rolls while there is an ally within 5 feet of them.

Corpse Explosion: Whenever the undead is reduced to 0 hit points and fails its undead fortitude, it explodes. All creatures within 5 feet of the zombie must succeed on a DC 16 dexterity saving throw or take 10 (3d6) necrotic damage, halved on a save. The creatures maximum hit points are reduced by the amount of necrotic damage taken until they compete a short or long rest.

Slam. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 8 (1d10 + 5) bludgeoning damage. The targets maximum hit points are reduced by an amount equal to the damage taken until the target completes a short or long rest.