Disclaimer: I do not own MLP FiM nor would I like to pretend I did.
AN: This takes place during A Canterlot Wedding Part 2 right after Chrysalis has knocked out Celestia.
"Shining Armor's love for you has made me even more powerful than Celestia!" The Queen of the Changelings proclaimed maniacal joy in every syllable.
"Perhaps," said an unseen pony from the shadow of an archway, "but that does not mean that you are unstoppable." Princess Luna stepped out into the light as she finished her sentence to stand protectively in front of her fallen sister. "Everypony has weakness little cracks in their power that can be used against them and I know yours Chrysalis."
"You? If you think you can defeat me you're sorely mistaken. Powerful you may be but Celestia was always the greater power and I have defeated her."
"Not when it comes to changelings. I assume you know your own legends." Chrysalis was looking scared now. "According to changeling mythology, a pony that has lost the pony they love most, that pony is protected from a changeling's power. You also know the story of the changeling's greatest victory."
"Oh yes I know this story that was a glorious day for the changelings. Wasn't it right before your little rampage that my ancestors took him from you forever?" the Queen's voice dripping with unsuppressed triumph.
The remaining guests whom had been trying to stay out of the way of the fighting began whispering amongst themselves the words "Who is him?" repeating and echoing around the room, curious as to whom the queen was referring.
"Yes it was." Luna said quietly. But with more force she continued "But that is not all I also have lost a most beloved sister." The room was once again filled with mutterings.
One brave pony asked, "There was a third sister?"
It was Twilight who answered "A very, very, long time ago Princess Luna had a twin sister whose task it was to bring forth the stars every night. But for reasons unknown she disappeared and has never been seen since, or at least that's what the history books say."
"My sister Starbright did not just disappear, she died I know not how, her spirit now lives in the stars she was guardian over. It was she who helped me escape the moon and then the nightmare." Princess Luna told the room at large. She turned back to Chrysalis fury beginning to burn in her eyes "It was you and your demons that took them from me." In the Royal Canterlot Voice she continued "You will not hurt anypony else in my family!"
"We shall see." Chrysalis cried as she shot her green magic at Luna. The Princess of the Night stood her ground countering with her own blue magic. Not unlike Celestia had done mere minutes previous. The rays of light collided in between them. Slowly Luna's blue magic was forcing its way toward the now perspiring and desperate queen. It inched a long until, finally, it collided with Chrysalis and engulfed her in a brilliant blue light bubble.
Shining Armor's eyes changed instantly back to their regular blue, the spell had been broken. Through the power of princess Cadence's and his love the shield barrier was strengthened forcing all the changelings, except for their queen, out of Canterlot. Chrysalis moaned in defeat and slumped into a sitting position inside her prison.
Celestia began to wake up on the ground. Luna turned to her sister looking both relieved and worried. Chrysalis seized her opportunity. She broke free of Luna's spell and without warning shot a jet of sickly green magic towards Luna before flying out the open window the join her minions outside of Canterlot as she flew away she roared "You shall rue the day you interfered in my plans!" Twilight and Celestia both ran after her shooting spells trying to recapture her but to no avail. Disgruntled and disappointed they turned around to face the room only to see Princess Luna crumpled on the floor.