"Ha ha ha... Tell me, Celestia, what do you intend to do now? Will you sacrifice your sister to destroy me? Or perhaps you will allow me to escape if I release her?"
"I refuse to harm my sister, but I can't let you run away either. There's no other choice. Luna... please, forgive me for this."
Just something I threw together, hence the slight lack of effort on shading. This is how I imagined the final confrontation between Celestia and Nightmare Moon took place over a thousand years before FiM's story began.
Celestia and Nightmare Moon exchange a few words upon meeting face to face in the old Everfree Forest castle. Nightmare Moon tried to force a sadistic choice on Celestia, hoping to worm her way out of being backed into a corner. What she didn't expect was Celestia using the Elements of Harmony to banish her to the moon for a thousand years.
Constructive criticism and suggestions are always welcome!
MLP: FiM is © Hasbro
Background is a slightly edited screenshot from the second episode of the first season of Friendship is Magic.