Doctor Who: 10 Stories You Must Rewatch After The Power Of The Doctor
6. The Haunting Of Villa Diodati
Before the forced regeneration scene in The Power of the Doctor, the Master explains that, on Gallifrey, he inhaled the Cyberium, which was first introduced in The Haunting of Villa Diodati.
The Cyberium is an artificial intelligence which contains all of the knowledge and future history of the Cybermen. Its whereabouts was a focus in Series 12, until the Lone Cyberman appears to collect it in The Haunting of Villa Diodati.
This story begins with the Doctor and company arriving at the titular villa, which has been the location for some very strange events. Then, a figure in the distance appears, who is eventually revealed to be the Lone Cyberman, with his mission being to take the Cyberium into the future to rebuild the Cyber-empire.
After initially inhabiting the mind of Percy Shelley, the Cyberium leaves his body and enters the mind of the Doctor. That is, until the Lone Cyberman issues an ultimatum, and threatens to kill everyone in the villa if he is not given the Cyberium - forcing the Doctor to relent and hand it over.
It is then used by the Lone Cyberman and later the Master, who creates the regenerating CyberMasters we see on the train at the beginning of The Power of the Doctor.