Gelatinous Cube

The monster fills its entire space and is transparent. Other creatures can enter that space, but a creature that does so is subjected to the monster's Engulf and has Disadvantage on the saving throw. Creatures inside the monster have Total Cover, and the monster can hold one Large creature or up to four Medium or Small creatures inside itself at a time. As an action, a creature within 5 feet of the monster can pull a creature or an object out of the monster by succeeding on a DC 12 Strength (Athletics) check, and the puller takes 3d6 acid damage.

Even when the monster is in plain sight, a creature must succeed on a DC 15 Wisdom (Perception) check to notice the monster if the creature hasn't witnessed the monster move or otherwise act.

Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 10 (3d6) acid damage.

The monster moves up to its Speed without provoking Opportunity Attacks. The monster can move through the spaces of Large or smaller creatures if it has room inside itself to contain them (see the Ooze Cube trait). Dexterity Saving Throw: DC 13, each creature whose space the monster enters for the first time during this move. Failure: 10 (3d6) acid damage, and the target is engulfed. An engulfed target is suffocating, can't cast spells with a Verbal component, has the Restrained condition, and takes 10 (3d6) acid damage at the start of each of the monster's turns. When the monster moves, the engulfed target moves with it. An engulfed target can try to escape by taking an action to make a DC 13 Strength (Athletics) check. On a successful check, the target escapes and enters the nearest unoccupied space. Success: Half damage, and the target moves to an unoccupied space within 5 feet of the monster's. If there is no unoccupied space, the target fails the save instead.