A Medusa Reversal is a machine created by the Master which undos the effects of a permanent quantum lock. It recreates the biology of the 'living statue' to its original, organic state. Effectively, it has the same kind of purpose as a Chameleon Arch would.
After narrowly escaping the destruction of the entire Dalek fleet that he himself had helped to create, the Master teleported to the distant planet of Solitrex. However, the radiation slowly decayed his body, and he frantically scavenged the abandoned planet for a way of stopping the decomposition or even preserving him long enough to be revived.
Solitrex was a planet that made use of scrap metal and heavily industrial and clockwork machinery to survive. The Solitrons preferred to use puzzles and games when activating a device. However, the natives suddenly fled and were never heard of again - leaving the planet to drift in orbit, habitable but remaining empty.
The Master teleported in a giant tower made of steel, rusty iron and wood. Scratchings and cuts on the walls stated it was the 'House of the Lost'. The place was full of technology for the Master to use.
In order to keep check of any experiment, he implanted his own brainwaves into the computer as a form of 'virtual cloning'.
The Master, rapidly deteriorating drew up plans that would not only keep him sustained in a stone form, but also have the chance to be revived again. He used the Solitrons' penchent for puzzles to construct the device, so that any visitor who knew of their location would not seem suspicious. After that, he went about the entire tower, carving inscriptions that detailed how he was a 'god' to them, and his stone form was merely an idol, so that curiosity would take over.
After building the intricate and large machine which fused itself with the rooms it was hused, the Master inserted the device that created the permanent quantum lock (which he used on Krymulus) deep within the machine. The Master activated the device and was instantly posed in solid granite. The machine automatically disassembled and switched settings to reversing the effects so that a visitor would activate it and release the Master.
The computer that housed a digital personality of the Master still survived - guarding the House of the Lost, waiting for his physical form to rise again.