1,000 years ago, give or take

Celestia gently, hesitantly touched her sister’s shoulder as Luna's sobs echoed from the castle walls. “I can't tell you how sorry I am,” she said. “I know I can never make it up to thee...”

Luna spun about and knocked the hoof away. “Thou knowest nothing! If thou did, there would no way thou could do this to me!”

Celestia recoiled, but felt her own ire rising. “Eternal night, Luna! Is that truly what thou want? No light, no warmth? No life?" She thought she saw the shadow of hesitation on Luna's expression, and pressed her case home. “Everything and everyone starving, freezing in the dark?”

“I don't care!” her sister screamed. “None of that matters anymore!” Her eyes widened with a forlorn hope that she could make Celestia understand. “'Tia, I deserve this. After untold thousands of years being the good little sister, I deserve this! I need this! Why can't I have this? Just this. That's all I ask. That's all I'll ever ask, I swear! I'll do anything, anything at all, big sister, if I can only have this one thing.”

Celestia approached, tears streaming from her own eyes as she tried to nuzzle, but Luna stepped back away. Celestia shook her head. “Luna, we discussed this long ago. I swear, if there were any other way...”

“There is a way! Just let me have this!”

“I can't!” Celestia screamed back.

Celestia's volume took her sister aback momentarily, but Luna's mouth settled into a hard line. Her voice shifted from pleading to an icy mock-formality. “Then We really have no 'big sister,' do We? And We never really knew you.”

“Luna, please!”

“No!” Luna stamped her hoof. “No! You ask too much!” She turned toward the third pony in the room, a bat-winged alicorn stallion who stood nearby. “Stalker! The Elements, quickly!”
Luna glared at Celestia as the stallion walked toward them, reaching into a pouch and withdrawing a crown with six gems embedded in a heart shape. She then gaped as the stallion carried the crown over to Celestia instead. She took it as gingerly as if its very touch burned her and put it atop her head.

“Stalker?” Luna asked, simply, unable to handle this of all betrayals.

The stallion's voice carried with it all the pain in the world. “I am sorry, dearest Luna. I only hope some day thou will be able to forgive me. Or at the least, to understand that I truly do love thee.” His eyes shut, then opened as he forced himself to look at what he'd done.

Luna turned to Celestia, eyes wide as she saw the multi-hued glow surround the crown, “No, 'Tia! Wait! We can...” Luna's voice became a piercing scream as the radiance shot out and enveloped her. The light seemed to sweep her away, her anguished sounds fading slowly away.

Celestia fell to her knees, weeping. The stallion crept up to her, his urgency overriding all caution. “What happened? What did you do?” he asked.

“She's bound within the moon,” Celestia answered, removing her crown and eying sadly the empty sockets where the Elements of Harmony once lay. It makes sense that there would be a price. If only this had been the dearest one. “We can channel warmth and life to her, Star Stalker. She'll live, We promise thee.” She got to her hooves, but turned away from the stallion, her head still hanging. “We're sorry.”

She chuckled, the sound more a dry mockery of humor than anything else. “It seems like that's all We can say nowadays. 'We're sorry.' 'Forgive us.'” She lowered her head even more, tears splashing against the floor. “We're sorry,” she repeated.

He touched a gentle hoof to her shoulder. “This was not your fault, Princess. And even if it were... If I cannot convince you that you are blameless in this... I forgive you.”

Celestia heard the words of comfort, but they simply sent her sobbing incoherently as she was struck again with the enormity of it all. Her guilt. Her loss. Her solitude. If he had only cursed her it would have been a blessing, something to rally against; his kind words felt like damnation, and the grief within his voice only underscored her own. Next he'd probably ask her to do it to him quickly, to be free from the pain he was in, and she couldn't even bear to give him that. It seemed to her that she would never be able to stop crying, and it seemed to her that it was nothing less than what she deserved.

Author's Note

Luna shifts from "I" and "thee" to "We" and "you" as another stab at Celestia. Basically she's declaring them no longer family or even close friends.