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.

Ash Puff. The first time the zombie takes damage, any living creature within 5 feet of the zombie must succeed on a DC 10 Constitution saving throw or gain disadvantage on attack rolls, saving throws, and ability checks for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on it early with a successful save.