"Rarity, please, we can talk about this," Twilight pleaded, staring helplessly at her wife standing at the end of the tunnel. "Just put down the torch before you do something that we won't be able to fix."
"I can't, Twilight," Rarity whispered, turning her head away in shame. "I'm sorry, but this has to be done."
"You're talking about committing high treason, Rarity. I can't pardon you for this." She might have been the primary ruler of Equestria, but Twilight had never felt more helpless than she did right now as she watched Rarity float the torch over towards the barrels of gunpowder that had been smuggled into this tunnel.
"I know. I won't ask for your forgiveness, dear. Only your understanding." Rarity smiled, and there were tears in her eyes. "I'm so sorry it had to come to this."
"Why, Rarity?" Twilight shouted as the flames from the torch licked at the wicks lying on the ground. "I thought you agreed with me on this. I thought you swore to stand by my side, until death do we part!" Anger burned her. "You promised you would always love me!"
"And I always will," Rarity replied. "You have no idea how much anguish this betrayal is causing me, but you know who I am. What I stand for." She laughed, and there was no humour in that sound. "I always knew I would have to make sacrifices for the greater good. That's what it means to be the Element of Generosity, no? And a national hero, to boot. But then...
"But then, I married you, and I became a princess too, and do you remember what happened that day?" Rarity didn't wait for a response before continuing. "I swore, before all fo Equestria, to always put our nation and our ponies first, and that, if push came to shove, I would make undaunted the final sacrifice without hesitation."
The flames began to travel up their individual paths up the fuses to ruination. "How is this serving Equestria?" Twilight demanded, even though she knew they'd had this argument so many times before. "How many lives are you going to doom today? How many ponies are going to die when you blow up the Foresight Engine?"
"Fewer ponies than would be hurt by you spying on all of Equestria, I should think," Rarity replied flippantly, and how that tone galled Twilight.
"The Engine will make sure that no one ever hurts another pony or tries to take over Equestria ever again without us knowing about it." Sizzle, sizzle, sizzle. It was almost too late now for Rarity to put out the fires. The point of no return was too close.
"Twilight, dearest, darling, sweetheart." The doomsday clock ticked down as Rarity shook her head. "We've sung this song and danced this dance so many times already. I rather think it's pointless now. I'm fortunate enough that the ponies who approached me with this plot thought to give me a teleportation gem to get out of here once the explosions begin, but I'd suggest you run before Equestria is left without its greatest princess and the love of my life. The wards here are very good at stopping any sort of magical interference."
"Don't you dare say that you love me," Twilight hissed. "Not when you've just ensured that I'll never be able to see you again without a set of bars separating us." It was truly too late now. She could see the flames lapping at the tops of the barrels.
"I told you I would make any sacrifice I needed to serve Equestria," Rarity replied, and the resolve in her eyes was unlike anything Twilight had seen since she had said "I do" at the altar so many years ago. "Honestly, I can't think of anything more precious I could possibly give up than us. I'm sorry, Twilight, and I love you. Always and forever. Please remember that." She vanished in a bolt of teleportation, leaving Twilight alone with the fire and gunpowder.
Twilight turned and ran away as flames consumed her heart and burned her futures to ashes.