Shadow People (Extremis)

Shadow People was the name that the Twelfth Doctor gave to people that existed within simulated worlds, such as the one created by the Monks. From their perspective, the world they existed in was real but in actuality they were digital constructs. The Monks created the world to study Earth and to plan a practice invasion, to see what would happen if they invaded for real. 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", freeing themselves from the simulation.