Dark Magic

Bad idea.

Cheaply cast spells that, the more you cast them, the more they wanted to be cast.

Price has to be paid:

"Debilitating" is the wrong word. A dark-magic-using pony will be physically fine right up to the point they start rambling about cryssssstals and ssssslaves.

Dark magic eats at a pony's soul- or, rather, at the positive aspects of a soul. They get consumed by the magic, leaving only the negative traits of pony personality that fuel the magic. Those the dark magic builds up and reinforces, because above all else dark magic wants to get cast, again and again.

How much of this "wants to get cast" is actual will of the inanimate, and how much is perception bias on the part of the ponies, is hard to say. You could also say that a rock in the air wants to fall down or that the air in a balloon wants to get out, but obviously in our world there's no actual thought, emotion or willpower involved in either case. That's human perception bias at work. But in the pony world, it's not impossible for dark magic to have a will- when we know harmonic magic has not just will but at least one incarnation.

"Inspiration Manifestation" is THE episode that demonstrates how all this works, at least for my headcanon.