John Amherst is an avatar of The Corruption. He spreads disease to people and seemingly even objects around him, and has an affinity for insects such as ants and flies.
He is described as appearing around 40, rail thin, with watery blue eyes, unruly upright dark hair, and a "nervous energy" , wearing a too-big brown coat (MAG 36).
MAG 68: The Tale of a Field Hospital (c.1899-1900): The supernatural version of the book The Tale of a Field Hospital contains alternate chapters not present in the published version that feature a Private Amherst. He is a soldier in the Second Boer War who is constantly surrounded by flies, repeatedly dies of diseases, and spreads them to others.
Jon speculates that given the age of the book, this may be an an ancestor of John Amherst, but given his many deaths, possibly the same man. The narrator of the book also mentions the name shared with Jeffrey Amherst, known for using smallpox blankets as biological warfare, which Jon notes as "a connection to a very different sort of monster, but still one that has the trappings of disease."
MAG 55: Pest Control (2011): An exterminator is called to an ant-infested house in Bromley by a concerned neighbor. John Amherst, the owner of the house, shows up and attempts to strangle the exterminator, who holds a lighter to Amherst's arm in self-defense. Amherst ignites very quickly, and the burning smells like burning flesh and rubber with an underlying smell of sickness - which the exterminator notes is the same as the smell when Jane Prentiss (another Corruption avatar) was cremated.
MAG 36: Taken Ill (2011): John Amherst takes over Ivy Meadows Care Home, where the residents become infected with a creamy yellow rash. The Care Home is later burned to the ground by Julia Montauk & Trevor Herbert.