Nearly hidden among the buildings, arches, and doorways of Sigil sits a cube, 10 feet on a side and composed entirely of mithril. Unlike practically every other structure in the city, this cube has no doors, windows, or other portals. It’s simply a solid block of metal with no filigree or adornments - with one bizarre exception.

Anyone who looks at the cube long enough will see what at first appears to be a shadow or reflection moving across its surface. Closer inspection will reveal that this is actually a sequence of numbers and letters that carve themselves into the mithril—a great equation that progresses across the face of the cube. The equation develops, moving at about an inch per second. It extends about 4 feet long, with the tail end constantly melting back into smooth mithril as the front end continues to build. It does not move in a straight line, curving from time to time for no fathomable reason, and occasionally splitting in two (or more) with each mini-equation following a different path for a while before they meet and rejoin a few minutes later.

One of the most interesting qualities of the equation is that it can be stopped. If someone puts a hand or other living body part in its path, the series of numbers and symbols stop their progression. (Unliving material has no effect on the equation.) Once stopped, the only way to get it going again is for someone to use a knife or other sharp instrument to carve out the next three numbers or symbols in the equation, after which the process will continue on its own again.

Scholars know that the equation is part mathematics, part arcane notation, and part divine symbology, but they cannot be certain what it means. Most believe that it is an academic notation for the state of the multiverse at that particular moment. They say that the secrets of reality can be found if one studies the equation long enough. So far, however, no one has come up with the answer.

The metacube is a planar touchstone, granting beneficial powers to those who can discern its mysteries.