"This event, sealed off in a corner of history, should never have existed."
Warning!
You are advised not to apply the following to other worlds, for it is unlikely to hold any bearing upon them.
Fujiwara no Mokou is a phoenix who was scorched by fate and suffers at the hand of her crimes. No matter how many times she is reborn, her sins will not fade.
You are a marebito, who came to us from a distant land. Several books make mention of similar occurrences, but the records are far too few to make sense of this all.
Resentment
She might not look it, but Mokou is the daughter of nobility. The Fujiwara family sat above the other noble families and was second only to the Emperor's house. So why would one of such privilege resent Kaguya Houraisan so fiercely?
Because of how her family suffered at Kaguya's hand?
Because she actually met Kaguya in person and found her detestable?
Because she was in need of something to obsess over?
All such reasons are plausible, yet none may be the case.
Ah, now I remember. At that time I was already an adult and had left that feeling well in the past. It was naive and nameless, but terribly important. That's it, isn't it?
That's how you felt, is it not, daughter of Fujiwara? When that rocket flew so high.
Monsters
The insanity of the moon turns humans into monsters. The magical power that rains down from the moon infests the world of humans with supernatural creatures. The Earth is stuck in perpetual conflict, with no hope of reaching the moon. Or so it seemed... (Hm, the continuation of this story will have to wait for another time.)
The Hourai Elixir
An elixir that grants immortality. It's clear, at least, that it did not originate on Earth, for its presence there brings chaos. It directly alters the supposedly unalterable laws of thermodynamics... It's both insanity and raw magical power.
The elixir embodies "fushi," or immortality, and played upon the "fuji" in the Fujiwara family name and the spiritual mountain's later name of Mount Fuji. The meeting with the manifestation of Mount Fuji's god also influenced the resulting insanity, as contact with a god of the opposing Water Phase triggered an adverse reaction. The Hourai Elixir is of the Wood Phase because it manifests life, and the concept of medicine is of the Fire Phase. There was fuel and the means to set it alight.
This fire is different, however, from the youjutsu Mokou learned many years later.
Fire
Fire is intense and impermanent. Once extinguished, it's gone forever. The antithesis of permanence, it's wild and momentary.
But fire produces earth. Ash plumes and falls, creating new layers. Those layers form a giant record that stores the history of Earth. The hakutaku knew this too. Furthermore, the "kagu" in Princess Kaguya is of the same origin as the "kagu" in Kagutsuchi, the god of fire and volcanoes, with both meaning "fire's light." They support her. There is power in helping other people.
Mokou's flame is not weak and fleeting, nor is it wild and chaotic. It is fixed in the shape of her memory and does as she commands. And I shall help her use it as the Legendary Mantle. This is my path, and I shall follow it until the journey's end.