Vampires

Everyone ready for a shallow dive?

Shallow because I ain't being paid to go look though books and dig up links, so it's from memory and thus imperfect, so I won't go crazy

So, vampire invites.

It all depends in where the vampires are from. Meaning that the real world myths of vampires are highly regional. That being said, Dracula is the real source of that weakness. It wasn't super widespread until the book.

Which means we kinda have to guess. See, the legends don't really say, and Dracula doesn't either.

The real origin of it in writing is Greek, and somewhere in the 1600s. Some weird shit someone wrote down that vampires couldn't enter without permission. Never gave an explanation afai could ever find.

So we're left with a couple of possibilities.

The first, and one that's commonly used in modern fiction, is that vampires are undead, and thus the living sort of imprint their abodes, leaving a kind of magic barrier that serves as passive protection against bad things.

Second, and it's not one I've seen a lot of in fiction, is that it comes down to building the house. That, intentionally or not, the act of laying foundations and placing doors/windows creates the same kind of magic barrier. Since old school masonry was sorta kinda mystical here and there, that may be the actual origin of the idea. Since some people did believe in magic to begin with, it makes a kind of sense.

Third, that it's associated with some of the "rules" of being a vampire, the changes it makes. Like spilling salt or rice and they have to stop and count the grains. Something about them as vampires makes them incapable of being so rude as to break in. Sort of a mental block, though in terms of believing in vampires, that could also be magical.

Fourth, it might be more about the tradition of blessings. New homes get blessed, or when someone takes over a home, it gets done. The holy finger waving and chanting is a form of magic, and thus the vamps are stopped by the spell/deity.

Now, it's obvious I don't think vampires are a real thing. I'm not even convinced that the myths have a basis in anything real that got expanded over time. But they're fun to work with.

Boring geek dump next.

My vampires in my ttrpg setting don't have this weakness because I thought it dumb. However, my fictional universe includes other types of vampire than my ttrpg setting does. My in-universe reason for some types of vampire not being able to enter is the intentionality of a residence. By being built, even without magic spells involved, the minds of the people doing the work shape the magic field and bind a degree of protection into the building itself. But, since people that live there are assumed to have the right to decide who comes and goes, it doesn't "turn on" until someone resides there. And it'll turn off once nobody does for a long enough time.