A gelatinous cube, also known as an athcoid, is a transparent ooze composed of mindless, gelatinous matter in the shape of a cube or rhombohedron. They are especially favored by the drow god Ghaunadaur.
Typically, a gelatinous cube measured between 10 to 15 feet (3 to 4.6 meters) on a side and weighed as much as 50,000 pounds (23,000 kilograms), although legends existed of much larger cubes. The creature was completely transparent, with only a glint of its surface in the light to give away its position. The occasional random object floating in a well fed gelatinous cube’s body could also serve as a warning of its presence to dungeon delvers.
Gelatinous cube bodies were remarkably moldable. They could flow around objects and through narrow passages, quickly returning to their shapes once enough space was available.
“The dungeon’s floors were spotless. That should have been our first clue.”
— Jaster Hollowquill, on his exploration of Undermountain.