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. Frequently the glint of its shiny form in the light was the last thing an adventurer ever saw.

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 the Abandoned Research Facility.