Gelatinous cube

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

Lairs
A gelatinous cube was an unusual, mindless predator that laired in ancient crypts and vaults. They roamed the halls of old ruins, scouring the floors and walls of any material in their paths. Organic material was 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 measured 10 feet to a side and weighed as much as 15,000 pounds. Cubes that fed regularly could grow far larger. It was only the occasional random object floating in a cube's body that warned dungeon-delvers of the dangerous presence of a gelatinous cube. Otherwise, the creature was completely transparent, with only a glint of its shiny form in the light being the last thing an adventurer ever saw.

Combat
A gelatinous cube approached any organic material it sensed with a slow, slithering pace. It attacked by touching its prey, which was then paralyzed with an anesthetizing slime that covered its entire body. The cube then engulfed the paralyzed target and dissolved the organic material in its body with powerful acid.

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

Variants
Some legends whispered of monstrous, "legendary" gelatinous cubes in the deep and secret places beneath Faerûn that had grown to several hundred thousand cubic feet of volume. Some bards spoke of gelatinous cubes formed from reproduction with a fiendish entity. These cubes possessed by rudimentary intelligence.