“Berenice” by Edgar Allan Poe
"Berenice" is frequently cited as one of Poe's most horrifying tales, and while its closing moments present one of the darkest images the man ever put to page, it is worth noting that it's an earned moment concluding a story full of comically misguided introspection. Most of it isn't actually a horror story; Mark Twain could've passed this off as his by rewriting the last two pages. That's Poe for you, unfortunately pigeonholed as a horror writer just because most of his stories ended in grisly fashion. This isn't his best work, but I have a soft spot for it.
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