One of the things I love about TV series such as Supernatural or Buffy is that they have their own mythology, their own way that monsters “work” within the confines of the setting. Movies do that too, but because they’re so focused on a single monster or a single premise, you rarely get to see it develop. There are exceptions, of course.
A demon in Supernatural works in a certain way, dies in a certain way. Likewise a vampire in Buffy. The audience is taught this, expects this, which – later, once this is established – allows the writer to mess with those expectations and create tension and drama in the process.
I want to do the same in my Dead of Night campaigns, to create a mythology of sorts that players come to recognise. So, when they come up against a vampire, they know it’s one of my vampires, and it will work in a set way. But I don’t want to bog the players down with loads of background and lore that the players have to go away and learn, or else tease them with as I expose it bit by bit.
Instead I want to create player buy in, to give them the power to shape this mythology for themselves. Of course, there’s a way to do this in the rules as written. “What’s This?” (p35) lets them specify a fact about the setting or a monster, whilst Vulnerabilities (p76) lets a player discover a monster’s weakness with an Identify check. But, for completeness, I’m going to put the two together:
Mythologically Speaking: A player can spend a Survival Point to add a new fact about a monster to the mythology of the setting. This fact is true for that type of monster from now on. In addition, the player who spent the point can exploit the fact in the game, perhaps gleaning a temporary advantage, discovering the creature’s lair or hunting pattern, or finding its weakness.
I’d quite like to find a way to reward players for reincorporating mythology back into the game, or perhaps for when the GM does so, perhaps by giving the player who first introduced it some sort of “ownership”, a permanent character advantage when facing down that monster, but I’m not sure how yet.
So, which monstrous mythologies are you fond of? Do you think players will help create a rich mythological tapestry, or wind up coming up with tenuous asides for mechanical advantages? And how might players (or GMs) be rewarded for reincorporating the mythology?
Steampower Publishing 
Shared creation always helps build buy-in, in my experience. A lot of it happens organically, as players make assumptions or discuss ideas that end up becoming part of the fabric of the game, but doing it explicitly is a good way to accelerate the process. This is especially helpful for one-shots.
Before introducing this to a group, I’d want to have a discussion to make sure that everyone has the same ideas about the tone of the game. It could bring a light-hearted game to a crashing halt if someone threw in a Survival Point and announced that the monster only fed on the eyes of children.
Some kind of veto process would help with this as well, so any player could feel comfortable in speaking up to say that they think a fact is too powerful, silly, dark, etc.
I can see a lot of facts being created for mechanical advantage, but as long as the colour that goes with them is interesting and they don’t end up making the monster a pushover, they will end up making the game more interesting anyway. If you think about vampires, especially in Hammer horror films, they’re vulnerable to a staggering array of things; this doesn’t stop them being scary, though, and the quest to exploit one or more of these vulnerabilities can form a large part of the story.