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. This feature can only be used once, after which the zombie must complete a long rest to regain it. 

Horde Tactics. The zombie has advantage on an attack role against a creature if the creature is unable to walk to another space on the ground because two or more of the zombie's allies are preventing it from doing so. (i.e. the creature is surrounded by hostile creatures, or against a wall or other imposing terrain feature while hostile creatures occupy all other available spaces.)

Multiattack. The zombie makes a slam attack. If grappling a target, it may also make a bite attack against that target. 

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (2d6 + 1) piercing damage. If the target is a humanoid, it must make a DC 11 Constitution save or become infected. The creature gains a point of exhaustion every 24 hours. The creature cannot remove points of exhaustion from resting. If the creature dies while infected, they rise in 1d6 days as a zombie. If the creature dies as a result of having 6 levels of exhaustion, they rise immediately as a zombie. 

Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.