Never Vault

The Never Vault was a location once visited by the Master and the Supreme Dalek around the slopes. They fought eachother at that point, however both survived the meeting.

When Clara Oswald (disguised as a Dalek) and Missy, the latest incarnation of the Master, visited Skaro Missy pretended in front of a Dalek to be prisoner of special significance. She asked to see the Supreme and told it to tell him that "the bitch [was] back". (TV: The Witch's Familiar)