The Horadric Cube is a mobile crafting device that is found in Act II Halls of the Dead Level III.
The cube functions differently in the Reign of Terror mod, then it does in Vanilla.
The first major difference is it can not be used as a "backpack" as it does not open that way. (Ex. You cannot place items into your cube for more "storage").
To gain access to the Cubes crafting UI follow theses steps
Find the Cube in the Halls of the Dead Level III.
Right click on the cube in your inventory.
A "Cube" will then appear under your characters feet as a "Crafting Bench".
Left click on the Cube at your feet to open the Horadric Cube Crafting Bench UI.
Do not worry about trying to pick it up when your done, as it never leaves your inventory.
It really is that simple!
The ONLY way to craft Runewords, Upgrade & Downgrade Gems and Runes is through use of the Horadric Cube.
Horadric Cube on the ground as a Mobile Crafting Station
Crafting Menu's UI. Botd anyone?
Showing that you have options to Upgrade or downgrade runes