So the theory is that all alicorn magic is evil. Like it’s just straight up dark magic - nasty corrupting stuff. That’s not to say that alicorns are evil, but just that their specific brand of magic is dangerous and corrupting.
There’s a couple of things that give this theory credibility.
Firstly, both Celestia and Twilight use this freaking evil looking dark magic at various points in the show. Celestia uses it during The Crystal Empire, and Twilight uses it during Princess Twilight Sparkle. What is interesting, however, is that when Twilight uses it in Princess Twilight Sparkle, Zecora specifically refers to it as “Alicorn Magic“.
The second thing is the Alicorn Amulet. This magical artifact apparently corrupts its owner and allows them to use super powerful magic. Possibly because it contains and allows use of alicorn magic by non-alicorns? Its name and the alicorn motif seem to suggest that, after all.
Sombra also uses magic which is similar in appearance, meaning his horn could be another alicorn-created artifact. It’s the same colours as the amulet.
Because of these I think that alicorn magic is actually really dangerous to use. It could explain why the princesses are reluctant to get involved with major conflicts in Equestria, and could also explain the whole Nightmare Moon thing.
Alicorns, being a combination of the three major pony species, are able to use unicorn magic too. So my thinking is that in their day to day lives an alicorn uses its unicorn magic, which is much safer to use. The alicorn magic would only be used in very dire circumstances, as it risks exposing the caster to the corruption.
Obviously this will never actually be canon, but I think it’s pretty cool to consider
There’s also Nightmare Sun in the card game. It’s not canon by any means, but if Alicorn Magic is truly as corrupting as this theory suggests, then it’s not so far fetched to think that Celestia could become Nightmare Sun.
@DragonWraith
Just a slight addition to both of you. Twilight was (still is?) the element of magic, so that could be the reason she was able to tap into alicorn magic despite not being a princess during the Crystal Empire episodes.
I’d never considered the ‘predestination’ thing, about how maybe she was destined all along for it to happen, but that’s kinda interesting too.
Then again, the way she became one is just as mystifying as this very theory, since she was either teleported in that plane of existence (that Celestia oddly enough was waiting for her in, without any explanation or reason given), or she could’ve been re-materialized.. Dark life vaporizing magic implications? sure.. but that at least would explain a lot more about her transformation and in addition, it wouldn’t be too far off then if that also leads viewers to believe that inherently, Alicorn magic is as this theory suggest, dangerous, it would only be second nature to becoming an Alicorn in the first place that the transformation itself is very dangerous too.
what I imagine is that, Celestia wanted this to happen as a last resort during the years she was alone, defending Equestria and understanding just how unpredictable the defense of the Elements is, meaning, Starswirl gets a little more depth and that spell he’d been working on makes a lot more sense too.. who wouldn’t want to have transformed alicorns in their ranks? we’d seen the kind of power they have in s4’s finale and that all could just be a dose of it, since Luna was even stronger as Nightmare Moon, as you said in the description OP..
there’s depth to this theory and I have to admit, I really like that about it.
I’ve always joked comparing the effects of dark magic on anything, but mostly normal ponies, to that of radiation poisoning on a healthy subject, and that only those of equal amounts of raw power could be able to handle it without much damaging.
Thus tainted artifacts such as the alicorn amulet would have the potential harming equivalent of the Elephant’s Foot.
Now imagine if it were broken by accident - How that malignant force would spread around.
That’s why she angry >.<