Viscous 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 cannot be seen but have total cover.

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 21 Strength check, and the creature making the attempt takes 21 (3d12) acid damage.

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

Viscous Form: The Viscous Ooze can alter its consistency from a solid-like state to a fluid-like state at will, making it harder to damage with physical attacks. When in its fluid state, it can slip through narrow spaces and avoid certain attacks.  It can also use its movement to change the shape of its body, though it still occupies the same number of squares - it can reposition them into a different area.

Dilatant Burst: If the Viscous Ooze is hit with a strong impact totaling 30 points of damage or greater, it temporarily hardens in the immediate area of the impact and then explodes outward, dealing 30 (6d10) piercing damage to all creatures within a 10-foot radius, or half as much on a saved dexterity throw beating a DC 18.

The Viscous Ooze makes three attacks per round:

Pseudopod. Melee Weapon Attack: +11 to hit, reach 5 ft., one creature. Hit: 21 (3d12) 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 18 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 21 (3d12) acid damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 42 (6d12) acid damage at the start of each of the cube's turns. 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 21 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the cube.

Shear Slam: The Viscous Ooze slams into a target with a hardened part of its body, dealing 22 (4d10) bludgeoning damage, pushes the creature back 10 feet, and potentially knocking the target prone if they fail a DC18 dexterity save. Melee Weapon Attack: +11 to hit, reach 10 ft., one creature. Hit: 22 (4d10) acid damage.

Viscous Tendrils: The Viscous Ooze launches a tentacle like pseudopod to grab a target up to 40ft away. On a hit, the target takes 21 (6d6) acid damage, is grappled (escape DC 20), and pulled 10ft closer to the Viscous Ooze. While grappled, the target takes another 21 (6d6) acid damage at the start of each of its turns and is dragged another 10ft per round closer to the Viscous Ooze, where it will be engulfed.  Ranged Weapon Attack: +8 to hit, reach 40 ft., one creature. Hit: 21 (6d6) acid damage.

Acidic Spray: The Viscous Ooze can eject a spray of its acidic substance in a 20-foot line. Each creature in the line must make a Dexterity saving throw, taking acid damage on a failed save, or half as much on a successful one.  Ranged Weapon Attack: +8 to hit, reach 20 ft., Line. Hit: 21 (6d6) acid damage.

 Absorb and Release: The Viscous Ooze absorbs kinetic energy from the environment, reducing incoming damage. As a bonus action, it can release this stored energy to increase the damage of its next attack by 1d10 or gain temporary hit points equal to 1d10 + its Constitution modifier.

Reform: The Viscous Ooze can use a bonus action to heal itself by reabsorbing parts of its own mass. It regains hit points equal to 1d8 + its Constitution modifier. 

Flowing Escape: As a reaction, the Viscous Ooze can flow away from an attacker, moving up to 10 feet without provoking opportunity attacks.

Absorb Impact: When the Viscous Ooze takes damage from a single attack that deals more than 15 points of piercing, bludgeoning, or slashing damage, it absorbs some of the impact and hardens, reducing the damage taken by half and storing the energy. On its next turn, it can release this stored energy as a bonus action, adding additional force to its next attack or using it to move an additional 20 feet.

The Viscous Ooze can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. The Viscous Ooze regains spent legendary actions at the start of its turn.

Ethereal Flow: The Viscous Ooze halfway steps into the ethereal plane and moves up to twice its speed without provoking opportunity attacks. As it moves, it can pass through creatures and objects as if they were difficult terrain. Any creature it passes through must succeed on a DC 16 Constitution saving throw or take permanent 2d4 STR damage.  This can only be recovered with a greater restoration or wish spell.

Devouring Wave (Costs 2 Actions): The Viscous Ooze surges forward in a tidal wave of corrosive fluid. Each creature within 20-foot forward of that direction must make a DC 16 Dexterity saving throw. On a failed save, a creature takes 21 (6d6) acid damage and is knocked prone.  All non magical items have been consumed by the Viscous Ooze. On a successful save, a creature takes half damage and is not knocked prone. The area affected by the Devouring Wave becomes difficult terrain until the end of the Viscous Ooze’s next turn.

Acidic Cascade (Costs 3 Actions): The Viscous Ooze releases a torrent of acid from its body, raining down upon its enemies for the next 3 rounds. Each creature within 30 feet of the Viscous Ooze must make a DC 16 Dexterity saving throw. On a failed save, a creature takes 18 (4d8) acid damage and is blinded until the end of its next turn. On a successful save, a creature takes half damage and is not blinded.  It must repeat this save if it starts its turn within this area of effect.