Horadric Cube (Diablo II)
The Horadric Cube is an item found in Act II on the bottom floor of the Halls of the Dead. The cube takes up four spaces in the player's inventory, but contains twelve spaces when opened. It can thus cleverly be used to increase the inventory size available to the player, although it's a little cumbersome to constantly open and close the cube to move items. Note that charms will have no effect on the player while placed in the cube, even if the cube is in their inventory.
The cube also has the useful ability of being able to transmute a host of different combinations of items when placed into it. There are many quite useful Horadric Cube Recipes available for use. Some of the most common include upgrading gems and runes to a higher quality, making normal items exceptional or elite and rerolling charms.
 Quest Items
Despite it's many uses, the cube is also a quest item required for two quests due to its transmuting power. The cube is first acquired during the quest The Horadric Staff, and must be used to combine the Amulet of the Viper with the Staff of Kings to form the Horadric Staff. The staff is then used to open Tal Rasha's Chamber.
It is used again in Act III, this time to unite the four parts of Khalim's Will: his heart, brain, eye and flail. The flail, after completion, is then used to smash the Compelling Orb in Travincal to open up the Durance of Hate.
The game constantly checks whether the cube is still in the player's possession after it is first acquired. This can happen if an inventory mod such as PlugY is installed, as the game can't locate the cube unless it's placed on the inventory spaces normally used by the regular game. If the cube isn't found in either the inventory or in the stash, the cube can be dropped again. The Council Members in Travincal tend to drop it under these circumstances. Note however that even if one has two cubes, they will both share the same inventory. An item placed in one can be taken out of the other.