From Diablo Wiki
The Horadric Cube is a powerful artefact made by the Horadrim brotherhood, capable of magically merging or transforming objects inside it. The box outer size is significantly smaller than the inside.
The Horadric cube features prominently in Diablo II, and you can read more about the Diablo II Horadric Cube item.
The Cube in Diablo III[edit | edit source]
The Horodric Cube is not in. Instead, Diablo 3 has Kanai's Cube.