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.

Jab. Melee Weapon Attack. The creature can attempt to deal 1D6 bludgeoning damage.

Hit of the Abyss. Melee Weapon Attack. The creature can attempt to deal 1D8 necrotic damage.  If the hit is critical, the target gains a new disease, every day their constitution score will drop by 1. Every 2 days, their maximum hit points will drop by 2.  If the constitution score or maximum HP is allowed to drop to 0, the target will die.