Framing story
Originally, I was going to have Twilight witness the events of the story through the past seeing potion thingy. I decided against it, though, because it made things a bit too complicated, and having Twilight see Celestia's past could cause problems later.

There are some funny lines in here, though.

Cadence and Twilight were walking around the Starswirl the Bearded exhibit when Discord appeared out of nowhere stared at one of the figurines.

"I must say, they didn't quite capture my beard right…"

Twilight lurched, turning towards Discord slowly. "What did you say?"

Discord rolled his eyes. "Oh, nothing!"

– – – –

Twilight looked at the potion, debating whether to drink it. On one hand, Discord's comment had made her curious. On the other hand, finding out would necessitate ingesting that substance again.

Twilight had long since decided not to investigate exactly what the substance was. She had a horrible suspicion that any investigation into what the substance was would necessitate finding out whose substance it was. And, quite frankly, even though there were only a couple possibilities, she didn't want to know.

Resigned to her ignorance, she finally decided to take a swig.

Celestia bragging
Originally, Celestia talking about the differences of the sexes included this exchange. It broke the mood a bit.

"Surely, after having almost as many lovers as me, you won't spout any nonsense about mares and stallions being identical."

Luna doubted at least part of that claim, but wasn't about to get into an argument about it now of all times. "I suppose you are basically right."

Original ending
I published this story with this ending, and realized it was probably too out of left field to work, and definitely too erotic to work.

Once Luna left the room, Celestia stood still for a minute.

Then, she collapsed to the floor in a fit of giggles.

It started off quiet, almost indistinguishable from sobbing, but it quickly gained speed and volume. Her laughter became louder and louder until it spread through the castle.

"Oh, you hate me! I love it when you hate me! I'm so glad I made this monster. It won't be long now! Come on, Luna. Become a monster! Be angry! Be vicious! Be brutal! Oh god, be brutal! I love it when you're brutal! Oh, Luna!"

– – – –

Luna heard Celestia scream.

She knew which scream this one was.

She winced.