Note: I will not be able to respond until tomorrow as I’m going to sleep after posting this.
I understand the concept of a magic that may be outlawed, by a town or even a nation, but how in the world do you get magic that is ubiquitously considered forbidden.
Magic in the AU ascribes to very little if any definition of morality. It is a tool similar to any weapon, be it sword or firearm, its morality is entirely dependent upon that of its wielder. So even if some magics were considered evil, it would be on the part of the creators of said magic. So yes we can have quote on quote evil magics, but is that enough to make it forbidden?
The main variable when considering this topic is of course, who forbade it in the first place. During the golden age there was no world government, and while the Olympians seemed involved in the lives of men they did not seem to be any sort of law enforcement on their part. So if not the gods or some nebulous governmental entity, what causes a magic to be forbidden?
Well I think the simplest answer, though lacking in evidence, is that the gods did forbid certain types of magic, either from being created or used. Most likely from being used, seeing as apparently not even the gods are aware of the full potential of magic, see Hades and his “pure dark magic” which is another can of worms entirely.
So what types of magic get forbidden? Well, based on the limited evidence we have, I believe it is any magic that seems to in some way manipulates spirit energy. This is because the two example of “forbidden” magic/spells are Death magic and Revival magic (I’m shortening it because I feel “dead revival magic” to be a bit redundant.) Both of these, in theory have some interaction with spirit energy. Death, under this theory, drains spirit energy leaving the victim a lifeless husk. Meanwhile Revival would do the opposite, returning spirit energy, or at least an imitation to a corpse (which actually has the interesting implication of the Peacekeeper having only magic energy in their body with no true spirit energy.)
Such magics would make sense to forbid by divine mandate, as death could be lethal against the Olympians due to it directly targeting spirit energy (something they ironically did not have to worry about, given that enough mages did them in just fine,) and Revival is the exact kind of nonsense that got Asclepius struck down (one of the many reasons Hades having influence on Acheron past the latter’s spirit form does not make sense, but that’s neither here nor there.)
So both of these make sense for the Olympians to forbide, but is this a good answer? I think so, based on what I know from both the lore doc and Greek myth in general it at least makes sense. Regardless I’d like to hear your guys’ thoughts about what makes a magic forbidden, and if you have any insight to what ever the Tartarus “Pure Dark Magic” is that would be much appreciated.