Undead Fortitude. If damage reduces the Zombie Giant Poisonous Snake to 0 Hit Points, it must make a Constitution saving throw with a DC of 5 plus the damage taken unless the damage is Radiant or from a Critical Hit. On a successful save, the Zombie Giant Poisonous Snake drops to 1 Hit Point instead.

Bite. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 6 (1d4 + 4) piercing damage, and the target must make a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.