According to Joan Acocella
In the summer of 1816, Lord Byron, fleeing marital difficulties, was holed up in a villa on Lake Geneva. With him was his personal physician, John Polidori, and nearby, in another house, his friend Percy Bysshe Shelley; Shelley’s mistress, Mary Godwin; and Mary’s stepsister Claire Clairmont, who was angling for Byron’s attention (with reason: she was pregnant by him). The weather that summer was cold and rainy. The friends spent hours in Byron’s drawing room, talking. One night, they read one another ghost stories, which were very popular at the time, and Byron suggested that they all write ghost stories of their own. Shelley and Clairmont produced nothing. Byron began a story and then laid it aside. But the remaining members of the summer party went to their desks and created the two most enduring figures of the modern horror genre. Mary Godwin, eighteen years old, began her novel “Frankenstein” (1818), and John Polidori, apparently following a sketch that Byron had written for his abandoned story, wrote “The Vampyre: A Tale” (1819). In Polidori’s narrative, the undead villain is a proud, handsome aristocrat, fatal to women. (Some say that Polidori based the character on Byron.) He’s interested only in virgins; he sucks their necks; they die; he lives. The modern vampire was born.
That’s the same Lac Léman summer when Byron wrote The Prisoner of Chillon (“Eternal Spirit of the chainless Mind …”).