OKAY GUYS WAIT I'VE GOT IT HERE IT IS
So, as I decided earlier, the Moon is sentient, but not the like in the movie, because it's not the Man on the Moon, it's just the Moon. It does perceive things, but not in a human way. Basically, it's there, but it doesn't care much for humans or anything at all. It simply watches and sometimes it has minor, or major, influences on Earth. They're kind of random, and don't really serve a purpose. It's comparable to solar eruptions, except, it's moonbeams eruptions, and they lead to magic happening here and there. The kingdom in which the AU takes place is where the Moon has had the most influence, and is often referred to as the "Moonlit Kingdom".
Flash forward to the Dark Ages, or rather it's equivalent. Pitch is a wizard living with his sickly daughter, Seraphina. She has been sick since birth, and after loosing any hope of curing her himself, Pitch resorts to pleading the Moon to do something for her. He does this every night of full moon, for years and years, with not much result. Still he keeps praying. Then one day, a terrible War starts and ravages most of the kingdoms. The Moonlit Kingdom is no exception. The Moon doesn't care for any of this though, and simply watches as humanity spirals into violence.
Pitch tries his best to stay away from the conflict and take care of his daughter, but ends up participating in the shadows, believing this is the best way to keep his precious Seraphina safe—which is actually kind of true. He ends up getting corrupted by it all, going as far as using Black magic and killing people in not-so-painless ways. Seraphina, of course, knows nothing of this. She sees her father disappear from time to time, and notices a slow but obvious physical change, as he is corrupted by his own dark magic every time he uses it. He still takes care of her, still loves her, still prays to the Moon every full moon, but he seems more and more distant as he slowly loses his humanity to darkness.
One night, on a full moon, little Seraphina goes outside ignoring the ache in her weak legs. She looks up to the Moon and asks it to help her father, who is turning into a thing of darkness every passing day. Pitch is inside by the window, not having noticed his daughter's leave, and just as he does each time, he pleads the Moon to save his daughter. The simultaneity of the wishes picks the Moon's interest and it decides, that all right, it'll do something, but just this once. So it sends a drop of moonlight down on Earth, to create a Moonflower. Only, the drop lands on Seraphina. Pitch witnesses this and watches, powerless, as his daughter is turned into a flower. The shock and pain snaps him halfway back to his former self, his greyish skin and rotten teeth, the only remains of his meddling with dark magic. Overcome by sadness, he takes the flower back inside with him. He stays locked inside, mourning, for days, weeks, he isn't sure anymore. He doesn't really care.
One night, he starts to sing his daughter's favourite lullaby, as a farewell. The flower reacts to the singing, to his greatest shock, and turns him back to what he physically was before the War. And Pitch suddenly understands. His daughter is still there and still alive. He keeps her hidden, until finally, after a few decays, the War is over. It is now safe enough to let her grow outside, letting her see the outside world she could not reach because of her illness, and plants her in a field. He cares for her, loves her, sings for her for decades and decades, centuries and centuries, until one day, she's stolen.
AND THAT'S HOW IT ALL HAPPENED FOLKS. Here, we have it. This is the final version of the curse and I ain't changing it. Never.
So, if anyone remembers, I mentioned Pitch needing the flower's healing powers for more than just keeping himself alive. Well this is it. He needs it to stay human.