Rassilon cube

Rassilon cubes were a method of incarceration created by Rassilon after the Hyperion War. They were the size of rubik's cubes, but were dimensionally transcendent. The cubes were made after the war left Rassilon with enough "so-called criminals" to fill a star system.

The Twelfth Doctor had a Rassilon cube in his TARDIS, which he used to trap the Fractures. (COMIC: The Fractures)