Corrosive slime. The sphere takes up its entire space. Other creatures can enter the space, but if a creature starts or ends it’s turn in it, the creature takes 1 D4 acid damage

Creatures inside the cube cannot  be seen and 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 12 Strength check, and the creature making the attempt takes 6 acid damage.

The cube can hold only one medium creature or up to four small or tiny creatures inside it at a time.

Battery acid. The acidic sphere takes double damage from lightning attacks. If a lightning attack hits it, the closest creature is hit by the lightning attack 

Pseudopod. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 6 acid damage.

Charge. The sphere can move into another creatures space. If it does so the target will have to make a D10 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 5 acid damage and 5 lightning damage.The sphere is then bounced 5 feet away in whatever direction it chooses, or can stay put.

If the target dies the sphere receives 20 healing and a 3 feet movement increase.