Extremis

Extremis was the name of an email that a virtual construct of the Twelfth Doctor sent to himself in the real world from a simulated world that was created by the Monks.

History
Within a simulated world created by aliens called the Monks, there existed virtual constructs of the Twelfth Doctor, Bill Potts and Nardole. When the Doctor was sent to the Vatican by the Pope, he began recording the events with the sonic sunglasses, since this version of the Doctor was blind too. After Bill and Nardole were deleted from the simulation, the Doctor sent the email to his counterpart to warn him of the Monks' intention to invade. (TV: Extremis)