It had started slowly. Luna's words to her had become annoyed, curt. She would question her judgment, where before she simply nodded her agreement.
Had she ever truly agreed? Or had she feared the alternative?
Celestia had ignored the signs. It was all a phase, she told herself. It would blow over soon.
It didn't.
She still hates that fateful night, when Luna finally rebelled. She still hates the jealousy she saw when that monster called Nightmare Moon consumed her sister. She still hates the fear and sorrow she felt when her incantation banished her to the moon.
Her subjects were distraught at the news of Luna's banishment. They feared what would happen if she were to escape. Would she seek revenge? Or forgiveness?
Celestia pondered those questions for herself. And as she thought, another one found its way in: would she show mercy if Luna asked her for it?
Despite her best efforts, the question would not be denied an answer.
She moved the moon for the first time that night. It felt so wrong, to do her sister's work, as if she was trying to replace her. As if she would never return.
It was then that she first saw the new collection of craters and hills on the moon's surface, all perfectly placed to form the shadow of a mare's head.
Luna's head.
It is said that Celestia's wails could be heard across Equestria that night.
Time can be a cold, bitter mare, for time causes ponies to forget.
It took nearly six hundred years for them to start. Those who had seen the moon hang unnaturally in the sky were long since dead. Historians began to write of the threat of everlasting night as a legend rather than fact. Mothers told their foals and fillies tales of Nightmare Moon to scare them to sleep.
And slowly, they began to forget Luna.
But not Celestia.
Her dreams replay everything. Every sign she missed, every word Luna spoke, every tear that fell from both Luna's eyes and her own.
Many night's she cries to the night, begs and pleads to whoever is listening out there to allow her to forget.
But time can be a cold, bitter mare, for she causes ponies to remember.
Without Luna, they began to elevate Celestia's status. They began to pray to her, swear by her name, idolize her actions.
In their eyes, she is a god.
But for all her power, Celestia is not a god. She never asked to be one. She did not decide who lives or dies, or ordain the flow of time, or cause good things to happen to her subjects.
And yet they praise her supposed deity. And after every celebration, she cries, for Luna's fears are coming true.
Even to ponies as powerful as Celestia, the gift of prophecy does not come voluntarily.
It most often manifests in dreams, preceded by great emotion. Through magic, the doors of time and space are opened, showing a chaotic image of what has yet to come.
For years, Celestia's dreams have foretold her sister's wrath upon her return. For years, she has feared tomorrow.
But now, tomorrow may change.
Her dreams now tell of a chance, a hope that Luna might return to her by the same magic that imprisoned her so long ago.
Her dreams also say that Luna's salvation will come at the hands of others. Celestia has not trusted others for a very long time.
But in the morning, she prepares, she manipulates, she uses; all in the hope that prophecy might one day be reality.
It is hard for her to trust. But in the end, it is all she has.
After a thousand years, the Nightmare had returned. And thanks to the Chosen Ones, so had Luna.
Celestia knows she has no right to ask for forgiveness. What she had done to her was horrible. Despicable. Evil.
But she had spent a millennium waiting, trying to fill the hole Luna left in her heart with something, anything. But like square pegs in a circular hole, nothing would fill it.
Celestia asks, and for a moment, she is afraid.
Luna cries, and runs to her sister, and at long last, the Luna-shaped void is filled again.