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John, often referred to simply as the Landlord, was the landlord of a large house in England.
In his childhood, his mother was dying of a terminal illness. One day, he found perculiar bugs in his garden and showed them to his mother, Eliza. Over night, the insects (revealed to be Dryads) came to life at the sound of high pitched frequencies, and healed Eliza by gradually transforming her into wood. Realising it was the only way to keep her alive, John lured various students to live in his house by allowing many people to stay whilst also being affordable so the Dryads could consume them and stabilise Eliza. Judging by the evidence the Twelfth Doctor found, he had been periodically harvesting university students every 20 years since at least 1957.
When Bill moved in with her friends in 2017, they noticed something strange about the landlord as well as the old fashioned and creaky house. With help from the Doctor, they soon found out his secret and convinced Eliza to end it, since she was parent and had the power to do such, as life isn’t really worth living in isolation. As such, she ordered the Dryards to eat her and John, and as a result caused all of them to swarm out of the house, removing all the wood and making the house uninhabitable. (Knock Knock)
At first, John appeared as a calm, collected yet discreet landlord. However, he quickly became more sinister and intimidating, such as when he sharply told Harry the tower was out of bounds and even throwing a menacing glare his way. He was quick to justify his own cruel actions in order to keep Eliza preserved by the Dryads. However, as events progressed, John eventually revealed his child-like tendencies, desperately wanting to save his mother by any means necessary. (Knock Knock)