Monk (species)
The Monks were an alien race who 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)
They were also able to see into the future through their simulation, and vowed to 'protect' the Earth from the devastation they foresaw would occur in one year. However, they required genuine consent from people with power, and were able to sense if their consent was true or they were acting out of fear or strategy. If their consent was not pure, the person would immediately be disintegrated. (TV: The Pyramid at the End of the World)
It was clear that the Monks showed no mercy for the human race in any way, and seeked to control the Earth and humanity itself. They had no problems instantly killing people, and on multiple occasions threatened The Doctor himself.
There was no clear hierarchy within the species, but it appeared that few Monks spoke while the rest remained silent.
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 cowls with various shades of red, hence their name. (TV: Extremis) They were apparently shapeshifters, choosing the look of a corpse at will. (TV: The Pyramid at the End of the World)
They spoke with a snarling, whispering voice, and their mouths did not move in sync with their voices. They typically moved quite slowly, and walked with their heads down and their hands together.
They were able to sense the purity of one's consent simply by placing their hand over one's chest. If their consent was not pure, then the Monks were able to disintegrate them. (TV: The Pyramid at the End of the World)
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)
They managed to make a 5,000-year-old pyramid appear out of nowhere between the three largest armies in the world by the 21st century - America, Russia and China. This pyramid appeared to be their base of operations on Earth and housed technology which enabled them to see into the future of their virtual Earth.
They were also able to use threads from their simulation of Earth to see into the future, and predicted a World War III-like event where all human life would be wiped out in one year. They were able to spy on the human race through CCTV cameras, and were also able to hack into the technology of normal citizens of Earth, such as mobile phones or watches, and change the time to issue warnings about the safety of the Earth.
It was later revealed the time changed in accordance with activities that unfurled within a science lab in Yorkshire when a scientist accidentally misplaced a decimal point and inserted 118.9 instead of 11.89 into the computer, killing their research immediately. The Doctor said that this event was going to 'blow up in their faces' and was the reason for the state of Earth seen by the Monks in one year. Also, the Monks could hijack vehicles and other technology and appeared to be able to teleport themselves into areas such as planes. (TV: The Pyramid at the End of the World)
