Celestia, for what it was worth, hated the caves beneath Canterlot. They smelt of dampness, and the reflective surfaces on the walls reflected eerie images of her as she strode down through the tunnels. But, for today and today alone, she had to be down here.
Above, she could hear the shouts and the the clamor as the battle raged overhead. She shuddered and continued on her way. As she walked, the beating of her hooves matched the beating of her heart surprisingly well.
The deeper she went, the closer it came. That hideous sound. It was overwhelming, overpowering, a complete and utter source of darkness. It beat like the drums of war, making the caverns rumble as one approached.
Celestia shuddered as she made out the first piece of the creature. The tail. It extended for hundreds of feet, spines jutting out of it, massive knives that reflected her face at hundreds of dizzying angles.
Celestia didn’t bother greeting the creature. Its cackling was enough to indicate that it knew she was here. “And so you arrive.” There was a terrifying slurping sound, like the creature was running its tongue across deformed lips. “I thought that the righteous Celestia was above consorting with the likes of me.”
Celestia nodded her head towards the ceiling, where the muted sounds of the chaos rang out. It let out a wheezing laugh, knocking a handful of stalactites down onto their heads. “I see.” Its tone was mocking, with an inflection to it that there was not a word for, one that rang with darkness and power. “The lesser of two evils, as it were?”
“No. Just the one most likely to save us.”
The wheezing sound started once more, the creature’s chest moving like a wave. The wheezing turned to coughing after a second, followed by spitting and the sound of something crashing into a wall. “Celestia, you mustn’t tell jokes,” the creature hissed, shaking the ground as it began to move its body.
Celestia made a silencing gesture and teleported herself to the creature's ‘head’, if it could be called such at this point. The smell of the now-dissolved wall was rancid, and it glowed with an eerie green color. “Rust, I need your assistance.”
Rust snorted, focusing its two, glowing red orbs on its head on the alicorn’s face. “Bah. Why should I? Millenia, caged beneath this city, with your people building empires on top of me?”
“Such is the price of war against harmony.”
The cackling resumed once more as Rust opened a pair of massive jaws, revealing the thousands of incisors inside. “Doesn’t seem to be the case with these changelings. Seems their queen and allies are giving you quite the fight.”
Celestia winced as she heard Luna’s angry shouts come from above, followed by the sound of an explosion. “You delay me.”
Rust’s eyes twinkled. “You waited all this time to come to me before. What’s a few more minutes?”
“Ponies are dying.”
There was the sound of stone breaking as the shadowy form of Rust pulled itself to a sitting position, up to the top of the ceiling, it head rubbing against it. “They die every day. You’ve killed them in this war of yours. Wonderful job.” There was a crash as its spiked tail landed within an inch of Celestia. “So tell me. Why I should help you?”
Celestia took a deep breath. “You’re the spirit of destruction. Aide me in this.”
Rust’s laugh was filled with darkness. “Celestia, Daughter of Harmony, Heir to the Heavens, the Sun Incarnate, comes to Rust, the Corruptor, the Eldist Evil, in search of aide against King Sombra. What a day to be alive.”
“Give me a damn spell,” Celestia growled. “I know you one. You have a thousand. If it interested you, you could wipe everyone off this earth in a heartbeat.”
Rust’s eyes, if that was possible, grew even wider. The thumping of its heart in that shadowy chest grew even louder, like it could taste what was coming. It brought its massive head, a shadow in the shape of some massive reptile, within inches of Celestia. It opened its jaws once more and spoke, letting Celestia smell its rancid breath, stinking of ozone and metal. “Be careful what you wish for, Celestia.” It said each word with deliberation, almost to the beat of its heart. “There’s a reason you burned every mention of me. There’s a reason I am down here. There is a reason that the last person to speak with me was a god.”
Celestia narrowed her eyes. “I know the risks. And I know that you’ll give me something. It’s what you do.”
The was a pause where neither of them spoke. The violence above them continued to rage, with the sound of fire, screams and more ringing out above. Finally, Rust smiled and shifted its form, causing stalactites to rain down once more. Celestia frowned, but a moment later there was a thud as a book landed in front of Celestia.
Rust sneered as Celestia levitated it with her magic. “Enjoy.”
Celestia looked inside, frowning. Words were scrawled across the page, like centipedes winding their way through tunnels. She turned the page, but was greeted with only blankness. “There’s only one spell here,” Celestia murmured, returning to the first page.
Rust cackled, tapping Celestia’s nose with a massive, shadowy claw. “For the time being. Only one for this situation. Another for when you need it. But now, all you need is the one.”
Celestia swallowed nervously, glancing from Rust to the book. “I’m not…”
Rust nodded upwards. “The alternative, however, is far worse for you. Pick your poison, as you might say.” There was a scream, louder than any before, followed by a thud and a buzzing. “Well? What’s it going to be?”
Celestia glared at Rust, but she began to read, murmuring words that had not been spoken since the world was forged. Rust just smiled, its eyes glowing like coals. Green lines flickered across the ground, emenating from where they stood.
After a few moments, Celestia closed the book, her eyes weary. “Well?”
“I’ve kept my bargain,” Rust said coldly, nodding upwards. “Now get out of my sight. I want to watch the show.”