“I will not allow it.”
Luna’s voice echoed off the smooth, marbled, walls of Celestia’s private chambers. The small, secluded room, located behind the throne room of Canterlot Castle was most often unoccupied. Celestia preferred to take most of her meetings in open court, usually to assuage any fears that she was abusing her power with secret backroom deals. Furthermore, being a ruler required maintaining a certain level of public access, lest said public accuse you of being out of touch, and open court allowed any pony with a grievance to petition their queen and know their concerns were heard.
But sometimes there were sensitive matters, matters that she wished to keep, not only the public, but her guards and castle staff ignorant of; for those she preferred her private chambers. She had set up and cast the noise canceling wards herself, as well as the selectively permeable barrier, so that she could be assured nothing came in or out of the room without her explicit authorization.
“Sister, we have no other choice,” Celestia replied, attempting to keep her voice level and free of anguish.
In contrast, Luna made no pretenses of being anywhere close to calm. “Are you so desperate that you would risk unleashing one hell upon Equestria, in the hopes of avoiding another?” she said using an accusatory tone that only a sister would dare to use with the ruler of Equestria.
Celestia’s voice cracked ever so slightly, and if anypony other than Luna had been in the room they wouldn’t have noticed it. “Yes, I’m desperate! A great terror threatens our land and this may be our only hope. Do you really think I would propose such an action if I was anything other than truly desperate?”
“You don’t know what you are asking of her,” Luna said softly, almost but not quite pleading.
“I am aware that I am asking her to do a terrible thing, but-”
“No you are not,” Luna said plainly but forcefully. Celestia looked at Luna slightly puzzled which the younger alicorn took as her cue to continue. “You say that you are aware, but the fact is that you have no idea the kind of temptation you are asking her to resist, and the kind of pain you will cause her if she fails. What right do even you have to ask that of anypony, much less one who has already done her duty?”
Celestia listened and offered her a thoughtful nod in return. “And yet in your protest you offer me no alternative. ”
“I do not have one, that does not mean I support your plan,” Luna responded quietly staring out the window, rather than meet her sister's gaze.