From Diablo Wiki
Act Two in the Halls of the Dead, a tomb under the Dry Hills, one of the early surface areas. You want it, so go get it as soon as you have the chance. Once you have it, the fun begins. If you haven't seen it, take a look at the picture to the right.
A full host of Cube Recipes are available in the game, in order to make Crafted Items, and is also used to get into the Secret Cow Level as well as the Pandemonium Event, some of the essential aspects of the game.
The Cube is one of the most interesting items in Diablo II, and functionality inspired by the cube is rumoured to also be included in . Check out the secrets of the Horadric Cube yourself!