Balstasse, Glutton of the Frog-Lords

Illusory Form. Balstasse has resistance to spell attacks cast by anyone attuned to a Staff of the Swamp Witch.

Engorging Self. Balstasse's attack damage with his giant tongue attack is proportional to the number of hit points he has, and can be calculated according to the table below:

Giant Tongue. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: X (see table) acid damage, and Balstasse gains 4d4 hit points. If the target is a creature, it is grappled. 

Swallow. Balstasse makes one giant tongue attack against a creature he is grappling. If the attack hits, the target takes the attack's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Balstasse, and it takes 21 (6d6) acid damage at the start of each of his turns.

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