Carmilla by Sheridan Le Fanu

In an isolated castle deep in the Austrian forest, Laura leads a solitary life with only her ailing father for company. Until one moonlit night, a horse-drawn carriage crashes into view, carrying an unexpected guest—the beautiful Carmilla. So begins a feverish friendship between Laura and her mysterious, entrancing companion.
But as Carmilla becomes increasingly strange and volatile, prone to eerie nocturnal wanderings, Laura finds herself tormented by nightmares and growing weaker by the day…
It is freely and legally available from Project Gutenberg: https://www.gutenberg.org/ebooks/10007