Burrowing Cobra

Tunneller. The cobra can burrow through solid rock at half its burrow speed and leaves a 5-foot diameter tunnel in its wake.

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

Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 8 (1d8 + 4) piercing damage. If the target is a medium or smaller creature, it must succeed on a DC 14 Dexterity saving throw or be swallowed. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the cobra and it takes 9 (3d6) acid damage at the start of each of the cobra's turns.
        If the cobra takes 15 damage or more on a single turn from a creature inside it, the cobra 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 cobra. If the cobra dies,  a swallowed creature is no longer restrained and can make their way out of the cobra with 10 feet of movement, exiting prone. 

Tail Stinger. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 7 (1d6 + 4) piercing damage and the target must succeed on a DC 13 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much on a successful one.