Aside from everything else, isn't magic supposed to be strange and mysterious anyway? There are always "Powers greater than man" in the game, and magic can be whatever you want it to be. In the case of an epic spell, that could mean a character...PC or NPC...could spend their lifetime (and using other magics, perhaps several) researching a spell. Or, they could get it from a divine/demonic/abyssal/other patron. A glimpse into the Far Realm gives a flash of insight perhaps?
D&D has always said that the rules are just guidelines which is generally the only thing the rules lawyers miss when they read the books. Something like this is a great example of a writer being creative and finding a way to make an important event in the story happen.
If your players really are that worried about whether the spell is "official" or not, all you need to say is that it's a unique spell/effect. After all, most epic magic comes about through great effort on the part of a spellcaster. Not all of them are willing to just turn around and share that work with the rest of the world.
D&D has always said that the rules are just guidelines which is generally the only thing the rules lawyers miss when they read the books. Something like this is a great example of a writer being creative and finding a way to make an important event in the story happen.
If your players really are that worried about whether the spell is "official" or not, all you need to say is that it's a unique spell/effect. After all, most epic magic comes about through great effort on the part of a spellcaster. Not all of them are willing to just turn around and share that work with the rest of the world.