Gelatinous cube

A gelatinous cube is a transparent ooze composed of mindless, gelatinous matter in the shape of a cube.

The average gelatinous cube measures 10 feet to a side and weighs as much as 15,000 pounds. The creature is completely transparent and dissolves living tissue with its acid.

Abilities
The following is drawn from the Monster Manual, 5th Edition, but some of its abilities have been demonstrated onstream.
 * Senses: Blindsight.
 * Condition Immunities: The blinded, charmed, deafened, exhaustion, frightened and prone conditions.
 * Attacks:
 * Pseudopod: The gelatinous cube can make a melee attack causing acid damage.
 * Engulf: The gelatinous cube can enter a Large or smaller creature's space to cause damage and suffocate them.

Known gelatinous cubes
The Mighty Nein encountered and defeated a gelatinous cube in Siff Duthar's abandoned research facility.

In the non canonical on-shot, the group of goblins stumble upon a gelatinous cube in the sewers of the Sandpoint town.