Oh boy I have a world in the making, but that story is not ready yet... don't know if that counts or it should only be for published worlds so to speak? If it counts I can make another post with facts about it and images ^^
Instead I have a dark fairytale, The Changeling's Sister, that I publish that resides in victorian times and in the world of fairies, so I can give you a lot of fairy facts:
In the story the main character's twin has been kidnapped by a fairy and replaced with a doll. In fairy lore this doll is called a fetch and is often times created out of a rotten log of wood or a rabbit's head or something like that, and the shadow of the victim.
These Fetches can also be refered to as changelings, but so can the person being kidnapped.
A fairy cannot lie or go back on a promise, but they will try and use it as much to their advatage as possible.
A fairy cannot enter a home without an invitation.
All fairies have green eyes, and are... I guess you can say hyperallergic to metals, especially iron.