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.

By creating and sustaining internal dimensions which were far larger than their external ones, these cubes could hold vast prisons within their small casings. Distorting the internal dimension of time to varying degrees within a cube could lengthen or shorten the duration of a prisoner's sentence.

The Twelfth Doctor recalled having been imprisoned inside a Rassilon cube for six months. After this, he kept a Rassilon cube in his TARDIS, which he used to trap the Fractures. (COMIC: The Fractures)