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.

 Charge. If the Undead Stag moves at least 30 feet straight toward a target and then hits it with a Slam attack on the same turn, the target takes an extra 10 (3d6) piercing damage.

Bite. Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 2 (1d4) piercing damage. 2(1d4) Necrotic damage

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