Gelatinous cube

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

Description
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.

Gelatinous cubes are immune to blinded, charmed, deafened, exhaustion, frightened and prone conditions. They can make a melee attack causing acid damage. Moreover, the gelatinous cube can enter a large or smaller creature's space to cause damage and suffocate them.

Known gelatinous cubes
In their second session of the home game, before Taliesin rejoined as Percy, that they fought a gelatinous cube and a mimic, disguised as a door.

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

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