Bandersnatch

Regeneration. At the start of each of the bandersnatch's turns it regains 20 hit points if its health is above 0.

Acidic Blood. When a creature within 5ft hits the bandersnatch with a melee attack it must succeed on a DC16 Dexterity saving throw or take 8 (2d6) acid damage.

Frumious Rage. When the bandersnatch is reduced to less than half its health, it enters a fiery rage. Its speed is doubled, it gains a +2 to AC, it gains an extra attack on each turn, and each of its attacks now deal a bonus 8 (2d6) fire damage.

Multiattack. The bandersnatch makes three attacks.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) piercing damage.

Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (1d10 + 4) slashing damage. 

Tail. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 12 (1d10 + 6) bludgeoning damage.

Chaotic Breath (Recharge 5–6). The bandersnatch exhales a line of force and necrotic energy that is 40 feet long and 10 feet wide. Each creature in that line must make a DC16 Dexterity saving throw, taking 17 (3d10) radiant damage and 17 (3d10) necrotic damage on a failed save, or half as much damage on a successful one.

Swallow. The bandersnatch makes one bite attack against a Medium or smaller target. If the attack surpasses the targets AC by 5, the target is also swallowed. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the bandersnatch, and it takes 7 (2d6) acid damage at the start of each of the bandersnatch's turns. A bandersnatch can have only one creature swallowed at a time.

If the bandersnatch takes 30 damage or more on a single turn, the bandersnatch regurgitates the creature, which falls prone in a space within 10 feet of the bandersnatch. If the bandersnatch dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.