The Vortex Cube is a very old and powerful item - a black cube that can be used as a container. If the eight Moonstones of Britannia are stored inside it, it has the power to draw and return very powerful objects from the Ethereal Void. Even without the Moonstones, the Vortex Cube is still powerful enough to allow teleportation throughout Britannia.
The Vortex Cube was created by Naxatilor to draw the Codex of Ultimate Wisdom out of the Void. After that was done, it was stored in their museum. However, it was stolen by human thieves. the Avatar found the Vortex Cube in the cellars under Stonegate and a cyclops claimed possession of it. Used together with the Moonstones and the lenses, the cube returned the Codex to the Ethereal Void.