Celestia had become annoyed at first, when Luna locked herself in the closet of her bedchamber. The first ten minutes, anyway. Now her little sister had been in there for an hour, and she grew more worried with time.

She could, at least theoretically, bash open the door with enough magic. Wouldn't take too much energy.

But that wasn't the problem.

"Lulu," she said, "if you could stop this silly business and come out, I would much appreciate it."

"No," said Luna from the other side, muffled, choked.

Celestia sighed. "Now Lulu—"

"And don't call me that!" The suddenness of the outburst took Celestia by surprise. "I'm not coming out, sister! Just leave me here!"

"Until what? Until when? This attitude of yours is not healthy. The Nightmare can't hurt you anymore. Do you understand that? Neither can the Tantabus. I thought that we've been through all this before. I thought we..." She wanted to say "trusted each other," but she wasn't even sure how much she trusted herself at this very moment.

For an achingly long time, nothing came from the other side.

Then Luna said, "I'm staying here," with shame creeping into her voice.

Despite this admission, Celestia tried to not let it get the better of her. "So you'll stay there until you die?" she asked calmly. "Starve yourself?"

"I guess so, sister."

The urge to break down the door only intensified, but Celestia knew it wouldn't do too much good. She waited for some kind of elaboration from Luna, maybe an explanation as to why she was doing this. Then she realized she wasn't going to get one, so she sat by the closet.

"It would be a terrible shame if you died," she said.

"No it wouldn't," said Luna.

She tried to continue acting collected about the situation, but Celestia was never exactly a good actress. 

But she tried.

"It would be a shame," she said finally, struggling to get the words out more than she'd like. "It would be a shame because I would have to bury you, dear sister."

"So what? You have to bury hundreds of your little ponies every year. Why should I be much different?"

"I know I have to bury them. I can't stop them from dying, no matter how hard I try. But I don't want to bury you too. I don't want to have to bury my own sister."

No response.

"Why are you doing this?" asked Celestia, flames of anger brewing in her chest.

"Because..." Uncomfortable silence. Luna thought about something, but Celestia couldn't tell what. "It wouldn't matter. Even if I told you, you wouldn't understand me. And if you don't understand me now, then you won't understand me in a week, or a month." She sounded more resigned than anything.

Part of Celestia wanted to do something terrible to her sister. She felt betrayed, like the pony she trusted most had done something secretive and wrong behind her back. Something treacherous, even. Yet she knew that her sister was in pain. "Very well," she said slowly. "I don't have to know what made you like this. Whatever is troubling you—whatever it could be—is what you can keep to yourself. I won't ask about it."

"Good. Then leave me be."

Celestia realized that she had a few choices. She could force Luna out, or she could let her stay, leave her to her fate. She didn't want her sister to die, and she didn't want her to be in pain. She knew that a pony in pain was a pony in need. But how could she help somepony who didn't want to be helped? Not just anypony, but the pony she knew most?

Celestia sighed. 

She wanted to cry, but she couldn't lose control of her emotions in such a way. "Fine," she said. "If you stay in there, that is up to you. If you come out, I'll forgive you. You won't even need to ask me. Just remember that I love you."

She couldn't do any more than that.

[hr]

Several hours later, in the middle of the night, Celestia had returned to her own bedchamber. She'd raised the moon, as she had to do during her sister's thousand-year absence, and let sleep overtake her.

So indeed she slept, albeit lightly—until she thought she heard something.

Seemed like the turning of a doorknob in another room...

Her ears perked up, and she awoke with a start. 

"Luna?" she asked, with hope, with desperation.