Crimson Splitjaw

Absorbent Carapace. Any time the Splitjaw is targeted by a spell of 3rd level or lower, it is completely unaffected and it instead regains a spell slot of the same level.

Magic Resistance. The Splitjaw has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Splitjaw's weapon attacks are magical.

Treasure Sense. The Splitjaw can pinpoint, by scent, the location of precious metals, stones, or magical items, within 60 feet.

Innate Spellcasting. The Splitjaw's innate spellcasting ability is Intelligence (spell save DC 18). The Splitjaw can cast the following spells, requiring no material components:

At Will: Control Flames, Frostbite, Shape Water
3/day Each: Melf's Minute Meteors, Pyrotechnics, Storm Sphere
1/day Each: Enervation, Immolation, Maelstrom

Spellcasting. The Splitjaw is a 8th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 18, +10 to hit with spell attacks). The Splitjaw has following Wizard spells prepared:
Cantrips (at will): [blade ward], [chill touch], [ray of frost], [shocking grasp]
1st level (4 slots): [catapult], [color spray], [mage armor], [thunderwave]
2nd level (3 slots): [hold person], [mirror image], [phantasmal force]
3rd level (3 slots): [counterspell], [haste], [vampiric touch]
4th level (2 slots): [Evard's black tentacles], [Greater Invisibility]

Bite. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 19 (4d6 + 6) piercing damage. If the target is a creature it is grappled (escape DC 17). Until the grapple ends, the target is restrained, and the Splitjaw can't make another bite attack

Tail. Melee Weapon Attack: +11 to hit, reach 10 ft., 1 target. Hit: 11 (2d6 + 6) bludgeoning damage.

Swallow. The Splitjaw makes a bite attack against a Medium or smaller creature that it is grappling. If the attack hits, the target takes the bite damage and is swallowed, and the grapple ends. While swallowed, the creature has total cover against attacks and other effects outside of the Splitjaw, and takes 21 (6d6) acid damage at the start of each of the Splitjaw's turns. If the Splitjaw takes 30 or more damage on a single turn from the inside of its stomach, it must make a DC 18 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone within 10 feet of the Splitjaw. 

Treasure Belly. When a creature is swallowed, they can make a DC 18 Dexterity saving throw. On a failed save the creature falls into the Splitjaw's stomach, on a success the creature falls into the Spitjaw's treasure belly (the creature has the same cover as when it is sent to the stomach). The interior of the treasure belly is approximately a 10 foot wide sphere and it's full of a liquid that kills bacteria and mold. A creature in the treasure belly is submerged in a clear poisonous liquid and the creature takes 13 (2d10) poison damage at the start of each of the Splitjaw's turns. If the Spitjaw takes 40 damage on a single turn from the inside of its treasure belly, it must make a DC 18 Constitution saving throw at the end of that turn or regurgitate all creatures and items from within its treasure belly, which land (creatures are prone) within 10 feet of the Splitjaw.

If the Splitjaw dies, a creature can use 15 feet of movement to crawl out of the Splitjaw, exiting prone. Alternatively, a creature can spend 20 feet of movement to crawl into the Splitjaw's treasure belly.