I always figured it had to do with alignment actually... not necessarily DnD style but that's a good marker because it involves multiple concepts.
Harry's Magical alignment varies throughout his career changing as he does in his style of usage. What I mean is, say SF he clearly gets a lightning motif, changing more fiery by GP,(let's see...) he gets more force magic in after the flamethrower, he'll fire from lasciel, ice from WK mantle, ect.
One caveat of magic(which just now I'm seeing a huge.. flaw? In harry being winter knight) is you are what you do and the more you do it the more it defines you.
Harry's motif in GP was fire, more so BECAUSE of the scene and it's related emotional response I think
Pyro fuego pyro. Can be translated with his intentions to do more. Say, HOT PASSION FIRE.(where were his other usages of it? We're they always a charged situation?)
He more or less, charged his magic with, himself, his soulfire. Uriel said he made him more of what he already was. I supposite, wizards can always use soulfire, the throwing of themselves into their spell completely.. but either by design or by concealment they weren't aware or couldn't break of that energy piecemeal, to access it is to throw the whole bottle..(which yes, I think does imply death curse wizards burn themselves into it)
An what Harry did might have been different because of his intentions, he didn't try to throw a death curse, he didn't intend to die an maybe, because he wasn't acting on instincts of destruction, it was in fact an example of the fabled opposite of a death curse, one born with intentions to create..
That he created destructive fire, is exactly why he was so torn by it. He had total control of it, it didn't burn him. The only other case we've seen of this is Justin
insert of theory Justin is the original unchanged timeline Harry..
which seemed to imply it was a matter more of control and belief(the two going hand in hand). The hardest part seemingly believing the disconnect from reality that is fire burns you.
Makes me wonder about 1 if NN domains can be so foreign because the owners idea of reality matches, al la Mab.
And 2 if Harry's immunity there isn't indicative of his position as a purifier(the nice way bad people say we're gonna destroy you lol), an agent of change and chaos.
Divergent thought...
🤔 If nic knows what's coming setting himself up, walking specific path, might be intentionally making him the shadow cast by Harry's light.
As far as the starborn aspect? Can't protect from yourself.. which is telling to me. He's a child of the stars and he specifically can effect outsiders because of this. The NN physically goes around the planet to about the moon per woj, so where are the stars? And what does being born to them have to do with outsiders? Is it the stars that we need look at, or the spaces between them..