"There's two ways most folks can cast magic, see. Either they meld with a magic critter in some capacity, or they can take bits and pieces of one and use rituals to focus the bits a certain way. The latter's leagues less risky, but you don't seem to be the type to go around tearing the wings off little fairies. You're a kind lass, you know. Fortunately there's ways around that.
With all these portals and thin spots opening up, see, not all of 'em open up to forests and deserts. Sometimes you get a place where the air is so thin and cold you'd freeze on the spot and puff up like a balloon. Or so thick and hot you'd be crushed and set alight. But sometimes, sometimes you pop into a place chock full of something that ain't quite water. Burns like dreadful acid, will take your skin right off. All this is why you gotta take a stick and poke it through any funny spot you find first before sticking your head in, mind. But if you stick a tap through that last one, put one end through the void and the other end in a good, thick glass bottle. Maybe even a keg if you've stopped it up well and good enough. You can get a good bit of it, and it'll work better'n most things for a spell. Considering what all else is magic, a lot of us old folks have taken to figuring it must be the blood or bile of something mighty terrible. But it's got so much of it, surely it won't miss just a little bit, right?
So you can take this stuff, and careful, careful so you don't burn yourself, put a drop of it on a good'n proper prepared spell ritual. And see, that stuff's the magic. Everything else you put on there is just telling it what to do. You put it on a paper with a picture and some spices, or a bowl full of leaves and critters, that just sets the rules. All the power is in that one drop.
Of course, if you cast your lot with a living critter, those rules and fancy bits go all out the window. But I'm not gonna dance around the facts and risk you doin' somethin' you'll regret. There's always some kind of catch that keeps people from jumpin' right to that in the first place. It ain't a hard rule that it has consequences, mind, it's just chancy.
Some critters ain't much more than magic, and they see you and you ain't much more than matter. So either by offer or by force, they take to jumping into you and kinda remakin' you both into a single beastie of sorts. Or maybe the magic bit in someone else breeds by have'n em stab a set of teeth into ye somehow. Now what the problem here is depends on what you got, maybe you lose your mind some of the time, maybe all of the time, or maybe you still keep your senses but now you've got a half a dozen things that can kill you. Ain't much use being magic if a ray of sunshine or a splash of water can kill ya. If you make out good and well you might get the magic and keep your mind without any real physical drawbacks. But then you're still a beastie, and now there's a bunch of mages that want you for bits and self-proclaimed knights that want to kill you just because.
Then you got the critters that are matter and magic already, and that gives ye a different problem. Because they don't need you. You're not necessary to them, you're just a toy. And how they treat you if they get their mitts on you varies about as much as how anyone else treats a toy. Maybe they'll treat you like the light of their lives, or maybe they'll slam you against a wall until your head breaks off. Maybe they break you by accident. Will you go on display, never see the light of day again? Or will they accidentally forget you on a trip and leave you stranded in some uncharted place? That's the risk you take so much as makin' them aware you exist, and it's why folks will get right crotchety if you try to get their attention on purpose.
Now, I'm no heartless old coot, you get mixed up with some critters somehow and I won't judge you. Better to get sweet talked by some beastie that ain't much in the way of good or bad than to get suckered in by some right cruel folks. But it ends nasty far more than it ends nicely, I'll tell you that much. That's why most of us who make it to a ripe old age play it good and safe, and most folks you'll meet that do make that bargain are young li'l whippersnappers. So I'm gonna teach you how to play it good and safe, and if ye ever do fall into somethin's hands I'll leave figurin' the magic part up to you. Because there ain't no amount of teachin' that'll learn you how to handle that sort a powers anyways."