Nightmare Moon slowly descends down a long flight of stairs. After several minutes pass she finally reaches the bottom as she enters a large room she is confronted by a cool breeze of air. There are only a handful of candles that don't even produce enough light for the entire room. The darkness in the room is so oppressive that one could almost reach out and grab it. It doesn't bother Nightmare Moon however, her eyes are just as well suited for nighttime use as they are for daytime.
This part of the castle used to be a wine cellar, back when it was still the Castle of the Two Sisters. Now it serves as a prison. Nightmare Moon's glum facial expression changes into a small but honest smile when she remembers all the times she used to sneak inside and secretly get drunk with her sister.
After a short walk Nightmare Moon reaches a large door guarded by two batpony soldiers. The door itself is made from thick steel plates with enchanted gems attached to them that absorb magic. As Nightmare Moon approaches, one of the guards opens the door and Nightmare Moon strolls inside.
This room is even more dark. But this darkness is not merely oppressive, it is almost sentient. Nightmare Moon can feel it grabbing hold of her and trying to drain her magic. Fortunately she is immune to it's effects because she made this herself to prevent the prisoner from using her magic to escape.
Nightmare Moon however is here for a reason and it isn't to test her security systems. She focuses on her horn, a small star appears at the tip of it. Then Nightmare Moon releases her spell and the star shoots forward and rapidly expands, driving away the darkness and lighting up the whole room in a magnificent white light.
At the end of the room, behind bars lays Celestia, the once proud princess of Equestria is laying curled up on a pile of hay that acted like her bed like a beaten dog, her rainbow coloured mane being the only colourful thing in this dull brick room. Nightmare Moon rears up on her hind legs and then violently stomps on the floor, the sound reverberating through the room.
"Wake up sister! We have much to discuss." Nightmare Moon announces matter of factly.
Celestia makes no effort to move.
"Why do you continue to come here?" Celestia speaks softly, her voice sounding just as broken as her appearance.
"Because even tho you committed many crimes against the batponies and you rose up against me when I tried to fix it. You are still my sister, and nothing would please me more then to have you at my side" Nightmare Moon answered kindly.
Now Celestia does move. She slowly stands up and looks at Nightmare Moon, her eyes radiating sadness.
"All I ever wanted was for my little ponies to be able to live their lives in peace and I have given everything to make this dream a reality. Yes I have made mistakes, the treatment of batponies being one of them. But!" Celestia's expression suddenly chances to one of absolute disgust.
"I have never been cruel, I never resorted to violence to solve my problems I never murdered those who opposed me, even when the communists in the north rose up and founded Stalliongrad and I was fully justified to resort to violence, I didn't."
Celestia moves even closer to the cell bars, her expression not changing.
"I will never sit by your side because you are not my sister! Luna cared about Equestria, about friendship and harmony. Luna would not lock me up in her personal recreation of tartarus."
Now it was Nightmare Moon's turn to glare.
"How dare you claim to not be cruel when it was you and you alone who imprisoned me on the moon for a thousand years!" Nightmare Moon all but yells.
"I only did it because it was necessary! And I have always regretted having to do so." Celestia says while breaking down in tears.
Seeing her sister in such a state, Nightmare Moon calms down.
"That is right sister, you did it because you thought it was necessary. Just like I know my transformation and revolt where necessary and how my Lunar Empire is necessary. The only difference between us is that I don't regret it because I know I am right. And I only put you in here for your own safety, there are those in my court that would rather see you dead. But if you repent and denounce the harmonic ways that have brought so much suffering to the batponies, then I can get you out of here." Nightmare Moon almost pleads.
"I won't do that. I will happily admit everything that I did wrong. But supporting harmony and friendship isn't one of those. And for as long as you take the place of my loving and kind sister, I will never even want to stand near you. And don't even get me started on your gang of violent thugs who don't know love, only death and destruction. I will never work with those who want to undo harmony. " Celestia speaks with a heavy amount of defiance in her voice.
After she is finished, Celestia turns around and lays down on her bed, her back pointing towards Nightmare Moon.
"Why can't you be proud of me! I am finally standing up for myself and making a difference, just like I did when I first approached you about the batponies. Fine, if you wan't to spend the rest of your life rotting away in here, then so be it!" Nightmare Moon shouts angrily.
Then she turns around and starts walking outside again. She doesn't see or hear it but behind her Celestia is quietly sobbing in her corner. As Nightmare Moon leaves the light slowly fades behind her. When she leaves the cellar she is suddenly confronted with her tiredness, she yawns loudly and decides that she has done enough work for today and needs to have some rest.