Horadric Cube

From Diablo Wiki

The Horadric Cube is an item found in the Act II of Diablo II in the bottom floor of the Halls of the Dead. The cube takes up four space in the player's inventory, but contains twelve space when opened.

The cube also has the useful ability of being able to Transmute, which combines the items in the cube to make a new item, if the objects form a compatible "recipe". An example would be combining three chipped gems to create a flawed gem.

The cube is aqcuired for the quest The Horadric Staff and must be used to combine the headpiece and shaft of a staff, so it can be used to open the Tomb of Tal Rasha. It is used again in Act III to unite the four parts of Khalim's Will.

The cube is considered a unique item and, if lost, can be reclaimed in the Halls of the Dead.