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.

Death Burst. The Zombie Burster explodes upon death, activating its burst trait. If killed by fire damage the resulting burst is instead an explosion, dealing Force damage. If the Burst is an explosion the radius is doubled however the dead will not return as Zombies

Burst. The zombie explodes and is destroyed. Each creature within 5 feet of it must make a DC 12 Constitution saving throw, taking 14 (4d6) poison damage on a failed save, or half as much damage on a successful one. A humanoid creature killed by this damage rises as a Zombie after 1 minute.