Shadow People (Extremis)

"Shadow People" was the name that the Twelfth Doctor gave to the virtual simulacra of people that existed within the Shadow World created by the Monks. He compared them to characters in a video game.

History
From their perspective, the world they existed in was real, but in actuality the people were digital constructs. The Monks created the world to study Earth and to plan a practice invasion, to see what would happen if they really invaded the planet.

In this world, a document called the Veritas became available and it was said that anyone who had read it had killed themselves. The Veritas contained information that informed the people that they weren't real, and in response they chose to "die", thereby denying the Monks the chance to study them further, and thus hopefully (as one put it) "saving the world".

The simulation contained versions of the Twelfth Doctor, Bill Potts and Nardole who all believed they were real too. After simulant Bill and simulant Nardole witnessed a "mass suicide" at CERN, they realised the truth and were both deleted from the simulation. The simulant Doctor faced the same fate, but was able to inform his real self of the Monks' impending invasion. (TV: Extremis)