This is the burial site of William Shelley. This is one of the many deaths that occurred in Mary Shelley's life. The death of her three-year-old son promoted the inspiration for the name and death of Victor Frankenstein’s youngest brother William. According to find a grave:

"Son of renowned writer Mary Shelley and poet Percy Bysshe Shelley. He died of an infection (malaria, cholera, or typhus), on June 7, 1819, while the Shelleys were living in Rome. He was buried there at the Cimitero Degli Inglesi, sometimes called the Protestant Cemetery. Percy and Mary were distraught at the death of William, who was the third child they had lost. Mary approached a nervous breakdown. Percy immortalized his son in his poem "To William Shelley". The exact location of William's grave is in question. It was reopened in January 1823 with the intention of interring there the ashes of his father, which had been stored for some months in a chest in the wine cellar of the English Ambassador. Gravediggers discovered the bones of an adult in William's grave and it was closed. Percy's ashes were interred in another spot, close to the resting-place of his friend John Keats”

This is the sad fate of a young boy, who died away form home. To be born in London and to die in Italy is within itself a bit of sad fate. This is part of the reason why Mary Shelley made Victor travel so much in her story. Frankenstein is reminiscent of Mary Shelley’s personal life.