From the windows of the Royal Observatory, atop the highest tower of Canterlot Castle, a magical glow of midnight blue streamed out into the dusk. For several seconds it remained strong, then faltered and eventually died. Moments later, a more intense radiance burst forth from the tower. That too faded quickly, leaving the stone walls again clad in naught but the dusk sunlight. Inside the tower room, a tall, regal silhouette stomped a hoof in frustration. Princess Luna stepped out onto the balcony and glared at the sun.
As she lowered her head, her horn burst again into light, and on the eastern horizon, a faint glow of a similar hue could, barely, be made out. Her face screwed up in concentration as the glow intensified, sparks of magical energy twisting free of the main force. Beads of sweat began to roll down her coat until, after a solid minute, the magic was extinguished and she collapsed, panting, onto the granite slabs beneath her.
It was no use. The moon would not rise.
The Princess took a few moments to gather herself. Then, standing tall once again, she spread her wings and took off. The sunlight seemed an insult as she flew in the direction of the Throne Room, the warmth on her coat and feathers only serving to increase her indignation.
"It is time for the night!" she raged to herself. "Why won't Celestia lower the sun?"
From the castle grounds, ponies looked up when they noticed the alicorn's shadow darkening the ground around them. The admiration they felt for the Lunar Princess was tempered, as always, by fear of her dark past as the dreaded Nightmare Moon, a fact which had grated on Luna for the past six years. They loved Celestia, adored her, practically worshiped the ground she walked on, the breeze she flew on, and the sunlight that she bestowed upon them. What did they do under Luna's moon?
They slept. They hid. They feared the darkness she brought them. It was necessary, yes, and they accepted this. But not once, in all the years since her return, or even before, had a single pony thanked her as they did her sister. And it weighed heavily on her heart.
As she neared the centre of the castle, Luna thought about what she would say to Princess Celestia. Would she feign confusion, or simply confront her outright about her failure to lower the sun at the appropriate time? She landed delicately outside the grand entrance and, as she moved to enter, the stained glass above the archway caught her eye.
The window was a beautiful depiction of the flag of Equestria, herself and her sister circling the sun and the moon, two halves of a whole. She pondered the strange ways time can separate ponies from those they love, and how this separation can change who they are. Luna thought back over the last few years since her return from banishment, how Celestia had eased her back into her daily routine after her rehabilitation. She remembered how stressful things had been for Celestia during that time, and since, and decided to give Celestia the benefit of the doubt.
As she entered the Throne Room, however, she was shocked to find it empty, devoid of her sister, or indeed anypony at all. As Luna walked towards the dual thrones at the end of the hall, she suddenly stopped dead in her tracks. The twin seats of power, carved from black and white marble centuries before, were gone. In their place, a single, massive, white throne dominated the dais. This new monument stood over ten feet in height, Celestia's emblem of the sun worked into the tip in gold.
As the Lunar Princess struggled to process her churning emotions, she heard the door behind her close heavily, and the metallic rasp of the deadbolts sealing. Whirling around, she caught the sight of a faint afterglow of golden magic dissipating from the locks on the mahogany door. Turning again, she saw a taller, while alicorn enter the Throne Room from the top, and pause by the oversized throne.
"Celestia!" Luna cried. "What is going on here? Why hast thou removed our thrones? Why hast thou refused to lower thy sun? Why hast thou locked the door?
"ANSWER US!!!"
Celestia looked slowly towards her sister, a soft hint of a smile on her face as she answered:
"Because I can, sweet sister. Because I can."
Luna stared at her sister in confused shock. Celestia let out a laugh - a short, harsh bark, far from the musical sound of her usual laughter - as she reclined on her throne.
"You seem surprised, sister. I assure you, this has been a long time coming. I have shared my place with you for six long, tedious years. Six years of helping you adjust, teaching you to walk again. I've practically had to feed you. And I have been met with faltering steps, infantile questions, and constant resentment.
"Don't try to deny it, Luna", the Solar Princess continued, as Luna opened her mouth to speak. "I ruled Equestria for a millennium without your help, and it was considerably easier. I gave you a chance to prove yourself again, and you have failed. You have failed your subjects, you have failed Equestria, and you have failed me. Your only sister. I am ashamed to call you such."
Tears welled up in Luna's eyes at Celestia's hurtful words.
"We have tried our-"
"We? Oh dear Luna, what 'we'? It's just you and me here, Luna, no one else! It's been six years, and you still can't adjust to something as simple as the modern vernacular. Your stupidity sickens me."
This hateful outburst caused Luna to physically flinch, and the tears finally spilled from the Lunar Princess' eyes as she sank into a sitting position, as though all her strength was suddenly sapped. Celestia smirked at the sight.
"Frankly, Luna, I see no reason to share my throne with one such as you any longer. From now on, I shall rule alone. I shall again assume charge of your responsibilities, since you have proven yourself incapable of leadership. I will be the Princess - the Empress - that Equestria deserves. And you will be what you deserve to be: nothing."
Celestia rose from her throne, approaching her weeping sibling. As Luna gazed up at her sister through her tears, she noticed that her eyes, usually indigo, were an almost luminous gold. Her surprise had not yet registered when Celestia spoke again, her horn beginning to glow an unearthly, terrible, golden magic.
"There is no place for you in my new Empire, Luna."