Elizabeth Bathory was 16th century, and she actually did it for real.

Then all these stories start forming, and create lore. Lore gets used for plotlines in modern stories.

The reason Elizabeth was bathing in and drinking virgin blood was because she was doing what a lot of royals used to do... ingesting blood of commoners to treat genetic blood disorders that come from severe inbreeding.

This is the origination of all the lore about blood drinkers and people using blood for anti-aging. Because royals did it all the time ever since they discovered that ingesting healthy blood was a treatment for their illnesses. The public could never know this, as their reigns always based their legitimacy on having the blessing of God behind it. How could someone chosen by God to rule a nation be a blood drinker who cant walk in daylight due to porphyria? It would destabilize their power in catastrophic ways.

This is a thousands year old globotrash secret.