Undead. The dog has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, stunned, or knocked unconscious.

Multiattack. The Undead Wolf makes two bite attacks.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage. 

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.