Details about how the Horadric Cube functions in Diablo 2 Resurrected. For information about how to obtain the Horadric Cube, check out the page How to Obtain the Horadric Cube.
The Horadric Cube is a 2x2 artifact that has interior dimensions of 3x4, making it a fine item to keep in your inventory for increasing your carrying capacity. While useful for holding odds and ends, note that charms in the Horadric Cube will not bestow any effects, nor can you use potions, scrolls or quest items stored within the cube. Also, Cain will not identify any unidentified items in the Horadric Cube - they must be removed, first.
The real purpose of the Horadric Cube, however, is transmutation. Put in several eligible items and hit the “Transmute” button and it’ll combine or alter them in some way. This is required to forge the Horadric Staff in Act 2 and Khalim’s Flail in Act 3, but it can also be employed to enrich yourself. Mixing three Rejuvenation Potions will yield a Full Rejuvenation Potion, three gems of the same type and grade will combine into one of the next higher grade (three chipped sapphires will transmute into one flawed sapphire), three runes of the same type can be transmuted into one rune of the next higher grade (three El runes transmute into an Eld rune), for example.
Other recipes abound, and the cube can be used for a variety of purposes, surprisingly not all involving turning three of one item into a superior version. You can use the cube to add sockets to weapons and armor, reroll the stats on rare items, to create arms, armor and jewelry with various set properties, to upgrade the base item type of unique arms and armor, repair non-ethereal arms and armor and more. Most of these formula require a base item, along with an assortment of runes and gems (usually of Perfect quality) to function. When transmuting items in the cube, you must only have the component items for said formula in the cube at the time, otherwise the cube will not transmute.