Confession Dial

A confession dial was a small, circular disk containing the last will and testament of a Time Lord. According to ancient Gallifreyan tradition, it was to be delivered to their closest friend on the eve of their final day. It was used to expose someone to their greatest fears and then make hem confess, before killing them. After being transported into his confession dial the doctor (being a time lord) managed to survive murder by The Veil (the creature inside) and burn his current body to create a new one as well as wiping his memory. He did this for billions of years.(TV: The Magician's Apprentice) (TV: Heaven Sent)

After being summoned by Davros, the Twelfth Doctor left a confession dial with Ohila for her to deliver to. (WC: Prologue) Missy brought it to Earth and met with Clara Oswald, and explained that the confession dial was to be given to a Time Lord's best friend prior to their death. Clara attempted to pick the dial up, believing that it was intended for her, but it responded by shocking her. Missy explained the confession was for herself, and would only open after the Doctor died. She took it with her to Essex in 1138 and to Skaro. (TV: The Magician's Apprentice)

Davros came into possession of the dial after he and the Daleks took the Doctor, Clara and Missy prisoner. When the Dalek city was destroyed by decaying Dalek mutants revived by regenerative energy, the Doctor took back the confession dial. (TV: The Witch's Familiar)

The Twelfth Doctor gave the Confession Dial to Mayor Me just before Clara Oswald's death in the trap street. He himself was subsequently teleported inside it, where he was pursued by a veiled creature from his nightmares and, after a recursive loop of dying and reloading himself from the hard drive that lasted for at least 2 billion years, he finally broke free and passed through a portal to Gallifrey. (TV: Face the Raven, Heaven Sent)