Willow's body has been improved with Solar Energy, that much is obvious. Achieving that kind of durability with mana would have been very obvious to me. The lightning was also based on Solar Energy, which supports that idea. I didn't get a chance to study her body, but it is made of flesh and bone, otherwise she wouldn't bleed. Which is very weird considering the fact that she was born a sprite, a mana based creature. It isn't impossible for mana based creatures to evolve into flesh and bone ones, but it is incredibly rare. On a surface level, a transition to using Solar Energy could be enough to warrant such an evolution, but there is no reason to make such a transition if you are already made of flesh and bone, with a few exceptions.

I highly doubt that Willow has become some kind of life mage, but there are other potential causes. I'm sure that I could, with a bit of effort and maybe some soul magic, figure out how to cause a mana based creature to evolve into a flesh and bone one, so it is reasonable that some other creature, maybe a dungeon or whoever that 'goddess' that Willow mentioned, changed her. I doubt it was a dungeon, most of them see themselves as being above the concerns of non-dungeons, and the few that aren't are only involved because they are weak enough to be threatened. And child dungeons wouldn't have the resources to pull something like this off.

It is also possible that Willow had someone teach her how to use Solar Energy in the same way I learned soul magic, and then she evolved into something with flesh and bone latter. But again, most of the creatures capable of that are dungeons, and Solar Energy isn't exactly good at that kind of telepathy. And she would need something similar in order to be that good with Solar Energy so quickly. And even then, she wouldn't be able to have such a durable body so quickly unless some powerful creature gave it to her or she was some kind of ridiculous prodigy, but if she were the latter she would be better at using it.

Obviously I can only guess at the cause, but lacking more information it seems pretty clear cut what is going on. Willow has some kind of patron, this 'goddess' of hers, that has granted her power. Why? I do not know.

Why does any of this even matter? Well, it could help us figure out how to better counter her if she shows up again, and she probably will.

No, that's not why. Well, it is a sound point, but that isn't why I care. I kind of hope that it is some powerful creature that twisted her body and mind to be opposed to us. It would make a certain amount of sense for her betrayal to be unwilling or at least magically coerced. It wouldn't make her actions forgivable just understandable.

And I really just want the her that betrayed us to not be the same person that I watched grow accustomed to being alive, whose soul I seeded, the child that I failed to connect with. I don't want that shard of innocence to have been corrupted.

And really, I just want my oldest child back.

Anyway, Good Night Diary.