The desire for a child. A desire that very few beings, no matter how incomprehensible they normally are, either don't understand or possess. Some with such a strong desire that they would go to great lengths to have a child of their own, or even just a surrogate. Sometimes those desires bring beings who, under normal circumstances would never meet, together. An example of such beings would be humans and the nightmarish beings collectively referred to as Great Ones. One such being is the Great One known as Flora, the Moon Presence. A Great One holding domain over the Moon and the Dream-World known as The Hunter's Dream. While it was, like many other Great Ones, sympathetic (in the broadest sense possible) in nature, it also wanted, like many other Great Ones, a child of its own (or at least a surrogate). Little did it know that it would get its wish granted when the Witch known as Lily Potter accidentally came into contact with the Great One through a kind of resonance, allowing the two to communicate, even if the Great One could only communicate back in vague and broad impressions and concepts rather than anything that could be properly comprehended by the Witch it was communicating with, like say, proper words.
The Witch who made the connection to the Great One apparently wanted the same thing as what the Great One wanted. A child. It was their mutual desire for a child (or at least a surrogate) that caused the two to make a mutually beneficial arrangement. In exchange for helping the Witch ensure she would become pregnant with her husband, the resulting child wouldn't be a human child but an infant Great One that would still be mostly humanoid in shape thanks to the Moon Presence making and implanting an organ vaguely resembling a human womb from its own flesh. If the child's human parents were to be rendered unable to take care of the child, the Moon Presence would step in and place the child in the care of its agents, a part of the deal that, while quite reluctant to accept, was probably the best offer that the Witch was gonna get.
Roughly ten months after the deal was struck between the Witch and the Moon Presence, the child was born. A healthy baby girl. The most notable signs that she wasn't normal (fully human) being a pair of comparatively massive wings, the wings resembling the wings of a bird (more specifically the wings of a raven (or crow)), and a pair of heterochromic eyes (the right eye being a pale red and the left eye being pale greenish blue) with black scleras. While the child's (human) father was understandably surprised and more than a little concerned, he ultimately came to accept the newly born baby, who was given the name of Fauna Lilium Potter. Even if he got quite upset by the fact that his wife struck a deal with a being beyond human understanding in order to finally have a baby behind his back. Needless to say, both of Fauna's (human) parents were quick to come to the conclusion that they couldn't afford having the knowledge of her existence become widespread. They didn't want to imagine what would happen to Fauna if the Truth of her parentage became well known, especially when she was still young.