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.

Lairs
A gelatinous cube is an unusual, mindless predator that dwells in ancient crypts and vaults. They roam the halls of old ruins, scouring the floors and walls of any material in their paths. Organic material is quickly dissolved in the acidic ooze that makes up the body of the cube, while stone and metallic material spend time floating in the body of the creature until eventually expelled.

Appearance
The average gelatinous cube measures 10 feet to a side and weighs as much as 15,000 pounds. Cubes that feed regularly can grow far larger. It is only due to the occasional random object floating in a cube’s body that warns dungeon-delvers of the dangerous presence of a gelatinous cube. Otherwise, the creature would be completely transparent, with only a glint of its shiny form in the light being the last thing an adventurer ever sees.

Combat
A gelatinous cube approaches any organic material it senses with a slow, slithering pace. It attacks by touching its prey which is then paralyzed with an anesthetizing slime that covers its entire body. The cube then engulfs the paralyzed target and dissolves the organic material in its body with powerful acid.

Reproduction
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.