Ooze Cube. The cube takes up its entire space. Other creatures can enter the 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 can be seen but have total cover. Each creature within the cube takes 9 (1d4 +5) poison damage on a failed constitution save DC 15. At failing DC 15 strength checks the poison damage increases to (4d8 + 5). 

A creature within 5 feet of the cube can take an action to pull a creature or object out of the cube. Doing so requires a successful DC 15 Strength check, and the creature making the attempt takes 15 (6d6) acid damage.

The cube can hold only one Large creature or up to four Medium or smaller creatures inside it at a time.

Transparent. Even when the cube is in plain sight, it takes a successful DC 18 Wisdom (Perception) check to spot a cube that has neither moved nor attacked. A creature that tries to enter the cube's space while unaware of the cube is surprised by the cube.

Pseudopod. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 15 (6d6) acid damage.

Engulf. The cube moves up to its speed. While doing so, it can enter Large or smaller creatures' spaces. Whenever the cube enters a creature's space, the creature must make a DC 14 Dexterity saving throw.

On a successful save, the creature can choose to be pushed 5 feet back or to the side of the cube. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.

On a failed save, the cube enters the creature's space, and the creature takes 10 (3d6) acid damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 21 (6d6) acid damage at the start of each of the cube's turns.

At the start of the engulfed creatures turn if they have failed two or more Strength checks to get out of the cube they begin to take (4d8 +5) poison damage as the toxin makes its way into the creatures system. When the cube moves, the engulfed creature moves with it.

An engulfed creature can try to escape by taking an action to make a DC 15 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the cube. 

After being engulfed by the cube for more than two rounds of combat they begin to take poison damage. The effects of this poison damage are not noticeable whilst they are still engulfed within the cube. Once free, have the creature make a DC 18 constitution saving throw. 

On a failed throw the creature, over a period of 1 minute, begins to feel an overwhelming heat beginning in the tips of their fingers and limbs. They will notice that the skin is melting away into a similar substance as the cube itself, the creature themselves becoming a Gelatinous Cube. On a DC 15 wisdom (perception) check or a DC 18 wisdom (medicine) check creatures will be able to identify that the engulfed creature is slowly and painfully becoming the Cube. 

Higher restoration, Wish or any other high level spell is required to stop the effects of this poison.