Gelatinous Cube

Ooze Cube The cube fills its entire space and is transparent. Other creatures can enter that space, but a creature that does so is subjected to the cube's Engulf and has Disadvantage on the saving throw. Creatures inside the cube have Total Cover, and the cube 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 cube can pull a creature or an object out of the cube by succeeding on a DC 12 Strength (Athletics) check, and the puller takes 10 (3d6) Acid damage.

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

Pseudopod Melee Attack Roll: +4, reach 5 ft. Hit: 12 (3d6 + 2) Acid damage.

Engulf The cube moves up to its Speed Walk without provoking Opportunity Attacks. The cube 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 12, each creature whose space the cube enters for the first time during this move.