[Inky]

Possibly not quite as dark as that. In old-fashioned folklore, pre-Victorian, the fairies generally seem to play the role of a wild card. They're not necessarily bad, but they have the potential to be highly dangerous if they feel like it. What will happen depends very much on the circumstances and the fairy, and in a game you could reasonably have fairies as either (doubtful) allies or (possibly convinceable) enemies. (I'd be inclined to not stat their powers, in a game, at least not all of them. They're often cast in the role of "Does obviously OP thing to advance the plot and then disappears again".) The implication that they somehow exist to be nice, or at least educational, to humans doesn't exist in the pre-Victorian versions. They're independent, neutral beings with their own opinions and a wildly unpredictable amount of magic power, and some of them can do very good things for you or very bad things. And which it'll be is very unpredictable - in some stories, there's a reason for it, in others it's just for amusement, or because of some arcane fairy custom that the human didn't know about.

They're also often portrayed as heartless or as not really seeing humans as people - a spiteful fairy might well kill or permanently maim a human just for fun, as a mean little boy might do to a frog.

Generally, someone finding themselves in that kind of a story should stay well away from these people, unless desperate - the potential rewards are big, but they're too risky. (Good game adventure material.) If you have inside knowledge of their society and what they might want, or some kind of magic that gives you an edge (fairy ancestry of your own, say, or "second sight" that lets you see more about what's really going on), the odds are more generous, but there's still no guarantee that they won't turn you into a pickled onion.

There are theories that these stories are garbled survivals of stories about the Tuatha de Danann (the legendary race that supposedly lived in Ireland before its current inhabitants arrived), or the Celtic gods. They make more sense from that perspective. These are meant to be beings who were here before humans were, and they think the land still belongs to them, and there is a serious risk that the land may agree with them.

I've heard the tithe thing in Tam Lin and Thomas the Rhymer, but those had it as a tithe of fairy souls, one in every seven years - but there was cheating. Again, the fairies are described as seeing humans as expendable if it came down to it. Tam Lin is kidnapped and trained as one of the fairy knights, with the intention of using him as the sacrifice when the time came. Thomas the Rhymer makes friends with the fairy queen and is taken to live at her court, but she eventually smuggles him away because the time for the sacrifice is coming up and "I fear, Thomas, it will be yourself".