All cubes have a common trait, they are cubes that have no natural explanation, because they were the first entities created, they forgot whether or not they are related.
It is like the Black Ball of Death in that it is pure black, though it has reflections. But the BBoD has many of them around the world, the OBFC is just one. It does nothing, but float and be ominous. It is undestroyable by any means, and people have managed to get on it, which is how we learned it seems made of a type of metal. It has side length of 120 feet.
It is still debated if these are related, though they are usually seen as less related than the balls.
The Gigacube: A giant magical cube of side length 800ft, it rolls around destroying everything it's face lands on, it also cannot be damaged or stopped. It avoids cities and towns.
Tesseract: It appears at a random location in the playground each day, you can ask it any questions and get the right answer.