The Monks used a virtual simulation of life on Earth in order to plan an invasion. Virtual constructs included simulations of the Twelfth Doctor, Nardole and Bill Potts. (TV: Extremis)
Biology
The Monks' appearance was that of a corpse-like humanoid. They had greyish blue skin, pointed ears and long fingers. They typically wore robes and hoods with various shades of red, hence their name. (TV: Extremis)
They spoke with a snarling, whispering voice. Their mouths did not move in sync with their voices.
Technology
The Monks had extremely advanced technology. One of their notable achievements was a computer simulation of the entire planet Earth in which they ran scenarios and practiced and perfected their invasion strategy, and also in order to assess the strengths and weaknesses of humanity. However, their simulation was too perfect. The simulations of real people that were part of the program were capable of gaining sentience. Once they realised the truth that they were nothing more than a program, they would choose to kill themselves rather than be constantly used and abused by the Monks. (TV: Extremis)