***
Celestia stormed out of the chamber. Twilight--her Twilight, sweet, insecure Twilight, had just stabbed her niece in the back. The sight of her adopted child running off in tears on a day when she should be nothing but smiles still swam in her vision. She was only vaguely aware of what she had said to Twilight. It had been unkind. She didn't regret it. Nothing could compare to calling the smiling little filly who liked flowers and playing dancing games with fillies "evil" and saying she was "trying to ruin her brother's life" on her wedding day. She was determined to leave Twilight in the grand hall without a second thought. She would make up with her when Cadance's spirits were restored.
It was then that she heard the explosion. She was distantly aware that there was a smell like charred fur, but her battle instincts had kicked in far before her senses. She was already opening the grand hall's double doors by the time she registered the smoke streaming from underneath them. Celestia crouched low, taking time to survey her situation. There were stained glass shards everywhere. She made note to watch her footing. No sign of current battle, which meant either the fight was at a pause or that there was an ambush waiting for her. She chose to find cover, rather than wait to find out. Which is to say, she made cover. With a flick of her magic, the floor beneath her erupted into a solid column of rock, which she was quick to hide behind. She peered from behind it, only now registering that amongst charred craters there was another pony in the room. A guard? Her mind ventured. Perhaps the attacker.
And then her heart dropped into her stomach. Lying on the floor, blood smeared across her coat and angry burn marks straddling her body, was her faithful student, barely drawing in ragged breaths. Something in the back of her mind dimly noted that she had at least three broken ribs, and that the slash on her head was magical in nature. Her previous reservations were forgotten. A loved one was in very real danger, and she would do whatever was needed to protect her.
And then her cover evaporated. A burst of magic and flames danced before her eyes, but she paid it no notice, waving it away with a shield spell. She scanned the area for an assailant, conserving her strength for her attacks. It was then that she saw Cadance, eyes glowing a horrible green, was hovering above her, a wicked grin on her face. "Surprised, Auntie?" The Cadance creature said in a voice very much not that of her surrogate daughter's.
Celestia said nothing. This creature was a threat. And not a threat the way Nightmare Moon or parasprites were threats. This creature was aiming to kill. Celestia's mind cooly sorted her priorities, and her first one was to protect her dear Twi--her subjects. She made a magical scan of the creature. Was this in any way Cadance?...No. The information widened her options for plans of attack. But just as she and the creature were about to fire, they were interrupted.
She heard a voice distantly call to her. "Bu...Princes...?"
She and the creature both turned to see Twilight attempting to get to her feet, coughing up blood as she did so. A look of disbelieving hope was shining in her eyes, breaking Celestia's heart and hardening her resolve all at once. The creature gave a malicious giggle, her horn glowing with a black-green energy again--
--And then screamed, as she had never screamed in her life before. Wings that once had been beating in even rhythm spasmed, the false Cadance flopping to the floor and trying in vain to ignore the shudders of agony that rocked through her body. Cold yellow magic crushed against her horn, a clacking, insectile noise filling the hallway.
"No."
The horn cracked, then splintered, and at last broke apart with a resounding crunch, and then the now-hornless creature's true form was revealed. Celestia registered a face in her mind's eye, noting that the creature had holes across its body and fangs in its mouth. She cared little for that. She would have it interrogated later. The creature likely had a purpose here, but that was secondary. The creature was murmuring something about the magic of love and her power, but Celestia took no heed.
She raced over to Twilight's side, healing magic surging through her body. "Twilight, Twilight speak to me. Are you alright?" Her ears flicked at the sounds of guards entering the room, and the scuffle of hooves on chitin as the creature was subdued. They may have been emanating at her from underwater, for all they did to draw her attention from her bloodied student. The unicorn gave a bloodied cough, fighting back tears of pain as the wound on her head closed up.
"I am now." Was all she managed, before she passed into a peaceful slumber in her mentor's arms.