Oxygen Independent. The Giant Space Oyster does not need to breathe. 

Massive Carapace. The Giant Space Oyster can swallow two medium creatures at once, provided they are adjacent to each other. Conversely, it can swallow one large creature. The Giant Space Oyster's stomach is large enough to hold up to two medium creatures or one large creature. 

Dissolving Stomach. Any creature swallowed by the Giant Space Oyster suffers 4d6 bludgeoning and 4d6 Acid damage at the start of their turn. 

Telepathic Spark. Any creature that ends it's turn within 30 feet of the Giant Space Oyster must succeed on a DC18 Int saving throw or become grappled. The Giant Space Oyster has such a strong willpower that it can prevent its foes from moving using only its mind.

Spit Up. If the Giant Space Oyster is dealt at least 20 damage to it's interior in a single turn, it must make a Con saving throw equal to (10 + 1/2 the damage dealt). On a failed save, it spits any swallowed creatures 30 feet away in a straight line. A creature spit up in this way that collides with a structure or another creature suffers 3d6 bludgeoning damage. If it strikes another creature, that creature also takes the damage. 

Multiattack. The Giant Space Oyster makes one Slam Attack and one Swallow Attack.

Slam. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 21 (3d8 + 6) bludgeoning damage.

Swallow. Melee Weapon Attack: +11 to hit, reach 5 ft. two targets. Hit: 18 (3d6 + 6) bludgeoning damage. This attack can only target creatures grappled by the Giant Space Oyster's Telepathic Spark ability. If it hits, the target is then swallowed by the Giant Space Oyster. 

Prismatic Spray (1/day). The Giant Space Oyster opens it's carapace and unleashes a brilliant cascade of damaging light. The Giant Space Oyster casts Prismatic Spray at 7th level. Swallowed creatures have disadvantage on the dexterity saving throw.

Scuttling. Whenever the Giant Space Oyster makes a slam attack on it's turn, it can use a bonus action to move up to it's speed. This does not provoke opportunity attacks from the creature targeted by the Giant Space Oyster's Slam attack.

Psionic Shield (3/day). When the Giant Space Oyster would be hit by an attack roll or a magic missile spell that originates from a source the Giant Space Oyster can see, the Giant Space Oyster can create an invisible barrier of magical force around itself that lasts until the start of its next turn. This barrier gives the Giant Space Oyster a +5 bonus to AC, including against the triggering attack, and prevents magic missile spells from damaging it.