This comic is based on one of the stories Pliny the Younger (the chap who, as a young man, witnessed the eruption of Vesuvius and wrote about it) tells his friend Lucius Licinius Sura in his letter (Book 7, letter 27 – oops, yes, there’s a typo on the original comic and header which has now been fixed on the comic, thanks Caroline L for the catch!), in order to get Sura to help him decide whether ghosts actually exist. You can read the whole letter, translated from the Latin, here.
Finally, as I couldn’t find a font I liked for Pliny’s handwriting in the comic, one that looked enough like Roman cursive whilst still being legible as English, I made one! If you like it, you can get it for your own projects.