The Machine

The Machine was a prototype TARDIS, predating the coinage of the name. It was Patience's husband's ship but she used it to get herself off ancient Gallifrey. (PROSE: Cold Fusion)

It was a creation of Omega. (PROSE: Cold Fusion)

The Machine was primarily a column, nearly a mile tall, with ten metre-thick cables spilling out of it. While mostly grey, some of its panels had been removed to reveal yellow and orange light sources. The cables and power couplings had grown out of the Machine in an organic fashion over the millions of years it had lain buried under the ice of a remote planet. At its base was a door to a small antechamber that functioned as a control room. The Machine was dimensionally immanent: it was smaller on the inside than the outside. (PROSE: Cold Fusion)

The primitive nature of the Machine caused violent ruptures in the Time Vortex where it had travelled, which drew in and immobilised the Fifth and Seventh Doctor's TARDISes. The Fifth Doctor and Patience attempted to heal this damage by remotely piloting the Machine back to Gallifrey to re-stitch the ruptures its original journey had created, but this nearly resulted in the destruction of Gallifrey when Provost-General Medford loaded the Machine with twenty-one fusion bombs capable of destroying all of Kasterborous in the distant past, convinced that the current crisis was the result of an invasion from their people. The Fifth and Seventh Doctors were able to halt the Machine long enough for the Seventh to trick the Ferutu into holding it in place, resulting in the Machine's past self colliding with it as it left Gallifrey, the collision destroying the future Machine and the bombs in the Time Vortex while the past Machine crash-landed in the past to fulfil its intended role in history. (PROSE: Cold Fusion)

Behind the scenes
The description of the interior of the Machine resembles the interior of TARDIS from the film Dr. Who and the Daleks.

The DWM illustrated previews show the Machine's outside as a pyramid, maybe as a reference to the original cover of The Beginning put up on the Big Finish website for the Doctor's TARDIS' original form before it was altered for The Name of the Doctor's depiction.