Melkur (The Keeper of Traken)

One Melkur, one of the evil creatures which were attracted to Traken by the power of the Source only to become calcified and immobilised when they entered its atmosphere, was in actuality a TARDIS used by, (TV: The Keeper of Traken) having been one of the most advanced types of TARDIS that existed on Gallifrey at the time. (PROSE: CIA File Extracts)

Uniquely both for a TARDIS and for a Melkur, the Master's Melkur TARDIS, after years of being tended to by Kassia, gained the ability to walk like a humanoid, though still appearing to be made of stone. The Master controlled him from the control room, which included a control panel on a room with a viewscreen connected to twin scanners in the statue's eyes, rather than the more common circular console.

The Master maneuvered events to get the Melkur (and, through him, himself) elected the new Keeper of Traken. To defeat him, Adric and Nyssa overloaded the Source, frying the Melkur with the feedback due to its connection to the Source as Keeper thereof. Although the Melkur was destroyed, the Master escaped unscathed in his main, non-humanoid TARDIS, which he had kept stored inside the Melkur's control room. (TV: The Keeper of Traken)

Behind the scenes

 * Geoffrey Beevers was credited as voicing the Melkur in The Keeper of Traken. However, in actuality, what appears to be the Melkur talking in the early parts of the stories is revealed to be broadcasting his own voice from inside the statue; assuming the Melkur was even capable of speech, there is no evidence that it actuality sounded like the Master.