Miasma
They gave me everything. All the free magic, the lives of everypony in Equestria—all in my hooves. My horn crackles, my skin bristles, and I can barely contain the incredible power surging through me... but I can't beat him. I can't save everypony. I can merely—
“It appears we are at an impasse,” Tirek says. His voice is confident and condescending, like he's already won. Tirek snaps his fingers, and all my friends—even Discord—appear in glass-like spheres around him. “How about a trade, Princess Twilight? Their release for all the Alicorn magic in Equestria.”
A numb, hollow feeling fills me, like my insides have vanished. What do I do? I can't just give Tirek the magic. It would be suicide. And I can't defeat him; the fields and mountains we destroyed are a testament to that. What will he do to my friends if I refuse? I can't move. I can't breathe.
My friends' lives are at stake. I can't let them die. If I give him the magic—Celestia, Luna, and Cadance's magic—everypony's lives will be forfeit. There's no guarantee he won't turn on me immediately and imprison everypony, or worse. He betrayed Discord, and he'll betray me. But what choice do I have?
“What's it going to be... Princess?”
How much longer will his patience hold? He's in control, and I can't fight him. Maybe he'll tire before I do, but how can I be sure? One wrong move, and he'll steal the power. Think, Twilight, think!
I can help you.
Who's that? This voice?
Suffice it to say that I am someone who can help you.
Everything slows down around me, like the voice and I can somehow communicate at impossibly fast speeds. I wonder how a disembodied voice could possibly help in a time like this. I must be going insane.
You are in an unwinnable position, Princess Twilight. But I can offer you a solution.
Who are you? What are you?
When the three Princesses funneled their great magics into you, parts of them went along. Traces of me have resided in one for over a millennium. When her magic left her, I left along with it.
But who are you?
My identity is unimportant. I was once a great entity, now reduced to a shred of essence no greater than a hair on your mane. I may be nothing now, but I feed on magic, on desperation, on hatred—all of which you possess in unfathomable quantities. Lend me your strength, and I shall make you a goddess. Tirek will be but a trifle next to you.
But I don't hate!
You do. You treasure your friendships, the love you have for your friends, above all else. Even that turncoat, Discord. And Tirek will take it all away—permanently. You want nothing more than to stop him, to destroy him. But you cannot achieve this on your own.
I... I hate Tirek. Bitter bile fills my mouth like I've just bitten on acid. How can I put all my faith in this voice? It's probably just a figment of my imagination.
It is your only hope, Twilight Sparkle. All other options lead to certain death for you or those you love.
My heart races as fast as it did in the moments when I learned I was to become Princess Celestia's apprentice; I was to continue my friendship studies in Ponyville; and those studies culminated in me discovering the secret to completing Starswirl the Bearded's greatest spell and becoming an Alicorn through the very friendships I'd forged. But it's not the same. It's cold, and I can feel nothing else.
Their cries sound like they're in slow motion. I can hear them, but I don't know what they're saying. Do I truly have no other choice? Can this voice truly be the answer?
If I am a mere result of terror-induced dementia, then channeling your power into me will have no effect, for I do not exist.
Oh, Celestia, I hope I haven't made a terrible mistake. I close my eyes and give in. I can't believe I didn't notice it before. The voice was right; it's smaller than a hair and almost invisible, but it's inside my mind, plain as daylight. I summon every reserve, every speck of magic I have. No spell is too small. With it, I channel my emotions—my overwhelming desire to defeat Tirek and liberate Equestria from his cruelty.
It's difficult. I can't do it.
Yes, you can. Concentrate! Let your desire to vanquish Tirek—and your hatred for what he has done—drive you.
I see it now. I can feel my power feeding the voice. The voice grows. It has an insatiable appetite, like somepony who's been starving for weeks and is finally having her first meal. It becomes a maelstrom inside of me, devouring all before it.
Like Tirek devouring the magic from everypony.
No. Oh, no. It's no vortex—it's a black hole. A source of pure darkness. My thoughts, my emotions, my magic, and the magic of the Princesses are becoming part of it. I can't stop it. It's eating me alive. Help me!
It's the Nightmare.
At last you understand. It amuses me how even the most intelligent of your kind come undone in the presence of something so simple, so primal, as panic.
It's consuming my soul. Every fibre of me is fueling it, making it more powerful. I can't do anything anymore. I can't move, I can't blink, and I can't even breathe. It's got complete control of my body. But it's not even my body anymore—I'm somehow on the outside, looking in. It's been perverted into something that should not be. My mane has been transformed into a dark nebula, my body pure black, and my eyes... my eyes... they're white, heartless lanterns.
Tirek takes a step back. “My, my, Princess Twilight, this is a new trick. Impressive.”
I hear him speaking, but I'm not truly there anymore. I'm an observer, a disembodied third party. I'm no more significant than the hair in a pony's mane.
“I choose the third option,” the thing that was once me says. “Your utter annihilation.”
I summon every ounce of power I possess. I have to fight this. I can't let the Nightmare take me as it did Princess Luna and Rarity. I have nothing. The Nightmare stole every drop of magic I had. I am its new vessel, and nothing can stop me. Tirek is the only other being in Equestria with magic.
“And my name is no longer Twilight,” the abomination continues. Its voice sounds nothing like mine—it is a thousand daggers in my heart. “I am the embodiment of a power beyond your simple comprehension. I am every dark thought within ponies' minds, every shadow where the light cannot reach, every harmful act. I have existed since this world was created. With the powers of its four rulers at my command, I have become the ultimate power in Equestria.
“I am Miasma.”
Tirek rumbles with laughter, the fool. He has no idea what he's up against. I don't think even he can stop it. Miasma, please. There has to be another way.
But you wished to save Equestria from Tirek. When I am through with him, he will not threaten anypony ever again.
Dark lightning crackles along Tirek's demonic horns. “There is only one ultimate power in Equestria, and that power is ME!” A sphere of orange and red energy forms between them.
He roars and sends the sphere hurtling toward Miasma. Miasma laughs—a chilling, mirthless laugh—and we dissolve into a thick, black haze that the sphere passes harmlessly through. Miasma, what are you doing?
Establishing my kingdom.
It's a weird sensation, being made of the magic smoke that no doubt inspired Miasma's name. Miasma—and I along with her—seep inside of Tirek. He tries to raise his arms to somehow block us, but his efforts are a waste of strength. We enter him through every pore. His essence and all the magic he'd stolen teems within him.
Miasma seeks out everything, as she did with me, and devours it—strips it from his soul. Even inside him, I still hear his screams of agony. The magic is unstable. Miasma's reckless flensing is making him too volatile. Then we gather at Tirek's core, becoming whole once more. It's too much. I only wanted to save Equestria, to save my friends. What have I done...? What have I done?
The only thing you could have: You have given me the ultimate vessel with which to fulfill my birthright.
Everything congeals and explodes. Tirek's body is no more. Stolen magic, blood, and chunks of his own flesh erupt in a geyser of destruction. A burst of white-hot magic fires from Miasma's horn. It ignites the rogue magic like gas vapour meeting a struck match. It's blinding and deafening, but we are somehow immune as it obliterates all in its path.
The blast fades, along my remaining will to live. Tirek and the surrounding countryside... and... Discord, Spike, Fluttershy, Rainbow Dash, Applejack, Pinkie Pie, and Rarity... all gone. Ash. That's all that's left. Ash, blacker than my heart, for leagues around. I didn't save Equestria. I condemned it. I signed its death warrant. My friends... I'm sorry... I'm so sorry...
I can't interfere or even communicate with Miasma anymore, but I can vaguely feel her tapping into her power. She accesses Luna's dream magic. She sends a signal, a powerful mental signal, to everypony in Equestria. She appears before them like she's really there, and she speaks.
“Greetings, my loyal subjects. The war is over, and Tirek is no more. I am proud to declare that you have a new Queen. I am Miasma, the ultimate embodiment of the Nightmare. The pony who was once Princess Twilight serves as my eternal vessel. At long last, I have achieved what has eluded me for so long.
“Welcome, my subjects, to the Kingdom of Nightmare. There is no friendship. There is no magic. There is only Nightmare, only despair. There is only Miasma.”
There is only Miasma... and now all I can do is watch her slowly destroy Equestria and everypony I've ever known and loved.
Forever.