The Monks

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.

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.
They spoke with a snarling, whispering voice. Their mouths did not move in sync with their voices.

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. (Extremis)
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Smartsuit

A smartsuit was a kind of space suit that provided oxygen in space by creating a force field around the wearer's head. It also provided a compact helmet to protect the wearer when in space.
At least by the series 12 version the suit had an AI that was programmed to deactivate the organic components — i.e. the wearer. Nardole recognised the interface as his ex, Velma which could be deactivated so long as the suit wasn't damaged.

They were programmed so provide air in terms of credits, the more credits one had the more breath they had. The default set of breaths appeared to range between 2500 to 3000. As Nardole confirmed, the more one breathed, if panicking for instance, the quicker they'd die. (Oxygen)

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Dryads

Lice, or "Dryads" - as coined by the Twelfth Doctor, were insect-like creatures which lived in wood and fed on humans. They lived in a house owned by the Landlord after he found three individuals outside, which had some healing properties that saved Eliza, however turning her partially into wood. In order to properly heal someone however, they had to be regularly fed people, so the Landlord decided to make the house cheap to lure people into buying it. He'd also use a tuning fork to call them to the area where the inhabitants were. In the end, Eliza controlled all the Dryads into eating her and her son in order to stop the killing that was keeping her alive, and as a result all the Dryads left.

Dryads bore a great resemblance to cockroaches. They had a chitinous hide, were a little bigger and had a brownish red color. They also had biolumenescent antennae on their heads.
They were extremely sensitive to high-pitched sounds and vibrations. This is how each Dryad communicated with each other. They could be controlled and tamed using music or a tuning fork, as demonstrated by the Landlord.

The Dryads were carnivorous and fed on prey, mostly humans, who they ensnared with the wood they inhabited. They would then swarm over the target and devour them in seconds.

The Dryads were intelligent insectoids and could sense when people were sick, such as Eliza. They would also cleverly ambush their prey unexpectedly and devour them. They could however be tamed and kept docile with high-pitched sounds like a tuning fork. Eliza also managed to control them mentally due to her mutation.

The Dryads had incredible healing abilities and could preserve a person indefinitely by transmuting their body into a substance with the same color and texture as wood. They would however have to be fed in order to maintain this preserved state or the person would revert to their original state and die.
The Dryads' natural habitat was the element of wood which an entire swarm could inhabit. They could also exert fine control over the wood they inhabited and animate it at a molecular level. They could effortlessly emerge or dive into the wood and animate any wooden object such as doors, shutters and trees. They would typically use it to ensnare their prey and then cause the wood to immobilise them or seemingly absorb them. They attacked their prey by swarming all over their body before breaking them down at a cellular level, effectively disintegrating the quarry. They could however just as easily reconstitute the body of a person they had disintegrated, fully restoring them to life. ( Knock Knock)
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Eliza

Eliza was the mother of John, the landlord.

In 1947, she was terminally ill and the doctors had given up on her. However, her son brought her what he thought were insects from the garden to try to cheer her up. They were in fact space termites. Once she was left alone that night, the sound from her music box awakened them and they began transmuting her body into wood in order to preserve her before she died.

When her son discovered the space termites' abilities, he tamed them and for the next seventy years would lure young people looking for accommodation to their house. The termites would then proceed to absorb the victims and transfer their life energies into Eliza to prolong her life. As he got older, John also told her that he was her father in order to prevent her from objecting to such an obscene decision, as the elongated life in isolation stunted her memory.
In 2017, Bill Potts and a few other classmates looking for student accommodation were invited by John into their house. Once the Twelfth Doctor, who was helping Bill move in, deduced something was amiss, the house proceeded to absorb Pavel, Paul and all the other students except for Bill and the Doctor, the latter of whom John had spared in the hope he could find a better solution to save Eliza.
Bill found Eliza first and witnessed a horrific display of the preservation process when the termites absorbed her friend. Eliza expressed regret over it but claimed she had no choice. John then arrived with the Doctor who offered his help.
 

 After he pieced together what was happening, Eliza remembered the Landlord was in fact her now aged son John and was utterly horrified by all the people he had killed just so she could live. John, angered the pair had revealed his identity, proceeded to summon the termites to kill the Doctor and Bill. Eliza managed to use her connection to them to make them disperse. She then hugged her son and caused the termites to devour them in order to put an end to the madness. As her last act, she also had the space termites restore Bill's friends as she had never fed on their life energies, thus saving them. The termites then caused the entire house to disintegrate.

Eliza was a kind and compassionate young woman. Though the preservation made her slightly naive and believe that the Landlord was her father rather than her son, she eventually remembered and was horrified and disgusted by the lengths he had gone to just to keep her alive.
She had also grown immensely lonely during the seventy years of her preserved state. This was a deciding factor in her decision to kill herself and her son as she reasoned that life was not worth living if she was isolated and locked away from the rest of the world. (Knock Knock)
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The Thames Ice Snake

Underneath the Thames, chained to the banks of the river, is an ancient creature of unknown origin. Alien or not, this giant sea snake has been responsible for the freezing of the Thames at regular intervals – none less so than during the last great Frost Fair of 1814!

Imprisoned by the ancestors of Lord Sutcliffe (since their discovery that the animal’s dung could be used as a superheating biofuel) the creature has a symbiotic relationship with a number of glow-fish in the river, which attract food and melt the ice to bring unsuspecting humans down into the water to allow the snake to feed. When the Doctor and Bill discover the creature, they become anxious to alleviate its suffering and find a way to free it… even if it means making a powerful enemy in the guise of Lord Sutcliffe.

The Doctor tricks the iniquitous industrialist by using his explosives to break the ‘Ice Snake’s’ chains, in the process preventing the death of the Frost Fair attendees (evacuated by Bill and the urchins), but as the ice breaks up, Sutcliffe is killed. With the creature freed, 1814 is the final Frost Fair on the Thames.

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