Gelatinous cube

A gelatinous cube is a transparent ooze composed of mindless, gelatinous matter in the shape of a cube. They slide through dungeon corridors, absorbing everything in their path, digesting everything organic and secreting non-digestible matter in their wake. Contact with their exterior can result in a paralyzing electric shock, after which cubes will proceed to slowly digest their stunned and helpless prey.

Gelatinous cubes reproduce through a form of hermaphroditic budding, in which a smaller cube is excreted in a side corridor, left to grow into full-sized cubes. These smaller cubes run the risk of being absorbed by their own parent on its next trip down the corridor. It has been determined that juvenile gelatinous cubes grow to fit their own natural environment, adapting their exterior proportions to the size of the corridors they most commonly sweep.

Variants
Some legends whisper of monstrous, "legendary" gelatinous cubes in the deep and secret places beneath Faerûn that have grown to several hundred thousand cubic feet of volume. Some bards talk of gelatinous cubes formed from reproduction with a fiendish entity. these cubes are possessed of a rudimentary intelligence, even able to take class levels.