"The usual way to do magic is--everyone has a piddly amount of their own. Enough to make a candleflame's worth of fire, or heal an injury not much worse than a papercut or a picked zit. If you put a lot of time into it, or work with other people, you can use it to make artifacts, which are less trivial.
Three thousand and some-odd years ago, the Ansati Empire had a magnificent magical infrastructure, including fountains of healing and youth. The Emperor--wasn't going to die, not naturally. But he wasn't immortal immortal. So his son the Crown Prince decided that rather than be a prince forever he was going to kill and usurp his father.
It didn't work. The country was destroyed by civil war, instead. But the thing is--the way he was going to usurp his father--there's a way for one person from my world to do nontrivial magic by themselves without taking weeks, and that's by fueling it with pain. Doesn't matter if it's yours or someone else's. So when his plans failed, the prince fucked off to a mountain and magicked himself a glorious castle, which he fed--and entertained himself--with the pain of people he kidnapped specifically for the purpose. I don't know where he got them from to start with, but for the past several centuries he's been getting them, one every few years, from the capital of Nathureme, one of the countries that rose in the wake of the Ansati Empire's fall."