kippurbird's inquiry was not posed in the form of a question, but is interesting all the same:
"I'd like to know more about merlins."
So would a lot of people, it seems, and it is because of that fact that I will now do as I am bid, and tell you more about merlins.
First off, merlins, small "m," are individuals with a tiny, tiny bit of fae blood—not enough to qualify them as a changeling, or cause them to look other than human-normal—who are nonetheless able to access some measure of fae magic. Merlin, big "M," was one of the first, and most notable, of these crossbreeds. Hence the term "merlin" being applied to the class as a whole.
So how much fae blood does it take to make a merlin? Not much. But if someone has too much, they'll be considered a changeling, which is a whole different ball of worms, and if they have too little, they'll be effectively human. Basically, if a faerie and a human have a kid, that kid will be a changeling (50/50). If that kid and a human have a kid, that kid will still be considered a changeling; just a very weak one (25/75). That kid's kids, however, will stand a very good chance of being merlins, as will their children. After two generations of merlins, sadly, you'll just get humans who maybe live a long time, or have a knack for finding fresh water. It's a short-lived gift.
(Yes, this has led to some families of merlins whose children marry each other, which can keep the magic alive for substantially longer, even if it does eventually lead to inbreeding and unpleasantness.)
Why do merlins matter? They're watered-down changelings, after all. They have little to no innate magic; they can't shapeshift or fly or teleport or do any of the other things that changelings and purebloods can do instinctively. So why don't the merlins just get trampled by their stronger relations?
Because they can use charms, potions, and written incantations, they aren't bothered by iron or bound by rowan, and they have none of the inborn limitations of true fae, that's why. A changeling or pureblood will always have checks and balances written into their very genetics, preventing them from accidentally destroying the world. Merlins don't have anything like that. They work their magic with external tools, and that keeps them from suffering from most of magic's nastier consequences. They are, in short, extremely dangerous. Many of them are also extremely devoted to gaining more power, more magic, more spells and tricks and charms. This can lead to some truly bloody encounters between them and the true fae, since a Puca's wings will fuel a lot of love charms. Not fun.
Most modern merlins keep their heads down and work primarily underground, since the fae are harder to find and faster to attack when actually troubled. But that can't last forever.
Someday, the merlins are going to present a problem.