I like the Castlevania explanation for why crosses work on vampires:
“Vampires are basically an evolved predator species, so their eyesight is pretty different to ours. Turns out that if you put a big geometric shape right up close in their field of vision, it confuses the shit out of their brains and, you know, makes them panic.”
This is similar to the explanation in Peter Watts absolutely brilliant novel Blindsight. The way vampire’s eyes evolved had given them phenomenal eyesight, especially at night, but they go into seizure when trying to process intersecting right angles, a genetic abnormality that wasn’t really relevant until humans started building structures and as people built more and more structures, the vampire threat waned, explaining their disappearance as well.
I cannot stress enough how brillant that novel is. Not only has it modern vampires done right, but also AI done right and completely alien aliens done right.
And the alien AI also talks like an LLM (and gets explained as such) written before anyone knew what the hell an LLM is.