Astral Devourer

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

Spell Absorption. The devourer can cast counterspell at will without any components. Its spellcasting ability is Constitution. Whenever it successfully counters a spell, it regains 1d8 hit points per level of the slot used to cast the spell.

Storage Sac. A creature outside the devourer can attempt to target its sac with an attack, taking a -5 penalty to the attack roll. If the sac takes 30 damage or more on a single turn, the devourer must succeed on a DC 16 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 devourer.

Multiattack. The devourer makes two attacks: one with its claws and one with its bite. It can use its swallow instead of its bite.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit:  18 (3d8 + 5) piercing damage, and the target is grappled (escape DC 17). Until this grapple ends, the target is restrained and the devourer can't use its bite against another target or use its inhale.

Claws. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 12 (2d6 + 5) slashing damage, and the target is grappled (escape DC 17). The devourer has two claws which can only grapple one creature each.

Swallow. The devourer makes one bite attack against a Medium or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the devourer, and at the start of each of the devourer's turns takes 10 (3d6) acid damage and must succeed on a DC 16 Constitution saving throw or be poisoned for one minute. At the start of a poisoned creature's turn, it must succeed on a DC 16 Constitution saving throw or be paralyzed until the start of its next turn. 

If the devourer takes 30 damage or more on a single turn from a creature inside it, the devourer must succeed on a DC 16 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 devourer. If the devourer dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Inhale (Recharge 5-6). The devourer unhinges its jaw and stretches it mouth to be almost as large as its body and begins inhaling. Each creature in a 10 foot cube originating from it must succeed on a DC 16 Strength saving throw or be swallowed by the devourer, as described in its swallow action above. 

Counterspell (3rd level spell, at will). The devourer attempts to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, the devourer must make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effects.