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.

Blind. During the transformation from a humanoid  into a Zombie Null, bone grows over its eyes and nose leaving it unable to see.

Echolocation. The Zombie Null can't use its blindsight while deafened.

Keen Hearing. The null has advantage on Wisdom (Perception) checks that rely on hearing.

Multiattack. For each attack that reduces a creature to 0 hit points, the zombie can make an additional claw attack.

Null Rush: The Zombie Null prefers to engage by sprinting at an enemy and slamming into them. If the opponent has a reaction available, they  may use it to Evade or Brace. 

Evade: If there is unoccupied space within 5 feet, make a Dexterity saving throw (DC 14) On a successful save; the opponent moves to an available adjacent space of their choosing while the zombie rushes past, ending it's turn. On a failed save, the opponent gets (-3) on the subsequent Strength saving throw.

Brace: Stand your ground, getting (+3) on the subsequent Strength saving throw.

If the opponent fails to successfully dodge; they roll a Strength saving throw (DC 16). On a failed save they are tackled to the ground and considered grappled, taking the difference in damage between their roll result, and the DC.  On a successful save, they remain standing while the zombie crashes into them, and the zombie takes the difference in damage between their role result and the DC. Then the zombie gets it's normal attack action.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:6 (1d6 + 3) slashing damage.

Bite: Melee Attack: +5 to hit, reach grappled, one target. Hit:6 (1d6 +3) necrotic. On hit, the target must make a successful Constitution saving throw (DC 15) or become diseased. The disease prevents  regaining lost hitpoints until cured.