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.

Tunneler. The zombie can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Multiattack. The zombie makes two attacks: one with its bite and one with its stinger.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the zombie. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the zombie, and it takes 10 (3d6) acid damage plus 10 (3d6) necrotic damage at the start of each of the zombie's turns.

If the zombie takes 30 damage or more on a single turn from a creature inside it, the zombie must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the zombie. If the zombie dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Tail Stinger. Melee Weapon Attack: +14 to hit, reach 10 ft., one creature. Hit: 19 (3d6 + 9) piercing damage, and the target must make a DC 19 Constitution saving throw, taking 14 (4d6) poison damage plus 14 (4d6) necrotic damage and be paralyzed until the end of the zombie's next turn on a failed save, or half as much damage and suffer no other effects on a successful one.