Darkel's TARDIS

 was a Tardis stationed aboard Space Station Zenobia around the time of The Valeyard's trial. It was used to transport Darkel and her security team down to the barren satelite orbiting the planet Etarho. In the rush to escape The Valeyard's trap The tardis was forgone in favour of The Doctor's TARDIS which was closer at the time. (Trial of the Valeyard (audio story))