I felt...different.

I could feel a bitingly cold wind all over my body, as I found myself lying sideways with snow falling to ground in front of my vision. Why was I in the mountains? I grunted as I got up, before moving a hoof to rub my forehead. As far as I knew, there were no bumps, bruises, or cuts; just my ethereal mane and horn wait what the fuck? Panicking, I grasped the sharpened spike in my hooves, looking around desperately for something reflective. From the corners of my eye, I noticed I was a lot taller, and also a darker shade of blue. I didn't bother with that, so I put all 4 hooves to snow and began to gallop. I had no idea where I was going, but it was downwards, so I was relatively satisfied with the route. I was going pretty fast as well: but speed is no match for military might when they find there's a new Alicorn, and is male. I'd probably be forced to marry some stuck up noble if that's how the hell the system works.
"What in the Sam hell happened to me?!" I shouted over blizzard conditions. My voice seemed a tad more into the G-major region, like a demon, and slightly deeper, not to mention more badass.
Ahhh, you're awake? Good.
MOONY?! WHAT DID YOU DO TO ME?!
Oh, a few 'upgrades' here and there, spit-shine on the other things, you know the deal...
BUT I'M A GODDAMN ALICORN! DID YOU MAKE ME WHAT YOU MADE LUNA?!
Mmm, suppose you could say that I did. Last night was rather wild, was it not?
...last...night...?
Of course, you were too busy with your vigilantism combining with my power to remember anything that happened. You went...how shall I say, one-colt Justice on Canterlot. Then, before you got captured, you teleported up here and passed out. So evidently, it was a wonderful night, which for some reason granted me masses of sexual tension. I don't think my lips have felt the need to ravage your mouth like that for quite a while, possibly since I first met you.
So I went kind of like a Green Arrow kind of thing? But, y'know, Marine Eyelander or something?
No, nopony died. It was probably since my powers don't have a tendency of blood-lust side-effects. But the new, outrageously sexy you means I can technically be in two places at once: Yours and Luna. And even then, I'll be able to exit you two and you'd keep the form for a few hours, and change back when I went in next.
So does this mean Storm Horror or whatever my name is can ravage Nightmare Moon? Sweet.
Exactly. And I can also exit you like I said, and have two of myself join in. That makes 4 of us in one session of sweet, sweet loving.
Nice. But how's everypony else in Equestria going to react to...this?
Don't worry, I'll close the doors.
No, no, not that. My new form. Will I be able to change back to Thunder when I want?
Mmm, not quite your choice, honey. I can change you to and from your Nightmare form when I decide. You technically get no say, but I'll be a good, sexy girl.
...so...what happens now?
Now? Hahaha...let's go practice medicine we show Celestia.

I flew through doors backwards, smoke pouring from my chest as I soared down a corridor without any control. Smashing hard into the wall, I could hear the classic 'breaking sentry gun' noise as my wings struggled to right themselves into loose ready positions. I winced as I picked myself up, wiping dust and blood from my fur, before the next attack came. A golden bolt of energy seared after me, before ploughing into a wall and reverting it to a melted gunge.
"Ah, shit." I murmured, smacking one side of my head to remove dust from in my ears. Finally, I put a hoof to my mouth with a frown, and called down the hallway. "Tia, would you just STOP? It's me, Thunder!"
"EXIT THUNDER AT ONCE, FOUL HELLSPAWN!" was the reply. Celestia stood in the doorway, eyes glowing golden with rage as terrified onlookers watched her slowly advance towards me with horn aglow. It was a standoff: Princess Celestis glaring down at what media would probably have named 'her most ancient and powerful foe'. Ahhh, nope, it's just me...Thunder Mustang. No wait, what was my shitty hero name? Marine Eyelander? Yeah, that. All I did was say hi. "Thou hath gone too far, Nightmare. Thou controlled my sister and now my good friend? Thou hath pushed thine luck."
"...what...?" I replied after a moment of silence in the corridor, from a few onlookers, Celestia, and myself. "Tia, believe me, if I was gonna be overtaken by a malevolent entity that grants power to its host, the Eyelander's be your sword to be angry with. Otherwise, I'm still he-"
"SILENCE!" Celestia boomed in the Canterlot Voice.
"NO U" I boomed back.
"SHUT UP, THUNDER, THIS IS BETWEEN NIGHTMARE AND I!"
"WELL, I'M STILL HERE, I'M HER HOST, AND MY THROAT HURTS FROM SHOUTING SO MUCH!"
"Let's stop shouting and continue this argument at dinner." Celestia sighed.
"Agreed." I replied, before we both left to get ready for dinner, and left onlookers confused as to what in the bat-shit had just happened.

"Soooo..." Twilight began on the train home. We'd all sat in silence as the carriage was rapidly evacuated due to my presence, and we sat in silence as they all awkwardly tried not to stare at me. "Marine Eyelander, huh?"
"Eeyup." I sighed, still looking straight ahead with a blank expression.
"Why's the fun stuff happen to Thunder?" Solar whined. He was reclined on his dearly-beloved Applejack's back as she read a book.
"Right place, right time, right marefriend." I snorted. "This kind of shit doesn't happen to Ebony, I don't think. I wouldn't know, I haven't seen him since he decided to take a political career under Celestia's guidance. Sadly, he hasn't responded to my letter about having the national anthem replaced with 'Elektro Sp00k', so that's unfortunate. But he's probably got that because he went for the goddess like I did."
"Except you also got her crazy side." Rainbow added. "And said crazy side gave you powers of...being tall and..." Rolling my eyes, I levitated a rifle maintenance book over to myself.
"Magic." I said, before snorting twice.
"But we must do something about that armour, darling." Rarity cut in, poking the Lunar Steel plates on my body. "Dark blue doesn't suit marine, I was thinking more along the lines of-"
"Gold?" Fire asked. Rarity giggled and nodded, before planting a kiss on his cheek. "I learned from the best." he chuckled.
"But hey!" Pinkie sharply jabbed in, popping out from inside my helmet which didn't surprise me at all, so I kept reading with a sigh. "If you're an alicorn this means you need a 'well done for becoming an alicorn' party! I'll invite the whole of Ponyville, and all your friends that didn't know you became an alicorn, but they might be scared of you so I'll..."

Meanwhile, in Thunder's mind...

Oh look, Pinkie's talking.
To me.
Oh look, the inner workings of a screwdriver sounds like a fun, 400 page chapter. Let's pay no attention to Pinkie and read about something that only the most pedantic man alive could write.

"...and there'll be cake and balloons and throw a spider and pin the tail on the pony..."
"Mmhmm." I murmured with a nod.
"...and Octavia and Cheerilee and Vinyl Scratch and Berry Punch and..."
"Oh, really?" I muttered. Everypony else had gone to sleep. I was amazed I was still awake after reading a chapter written by some pedantic twat who lives in a shed and writes book chapters about how a metal rod on a piece of plastic can unscrew something.
"...and hookers and strippers and booze and drugs and gunfights..."
"Mmm, sounds delightful. Hey Pinkie, I bet you can't hold your breath for 20 minutes."
"UHHH, I BET YOU I CAN!" Pinkie filled her cheeks with air, turned blue after about 20 seconds, and passed out. I smirked, and finished my chapter. Then, I moved onto the next one as the train continued its voyage down the mountain and to Ponyville. 5 hours to go, so I might as well busy myself reading about how a torque wrench is different from a spanner.
And for once, I didn't use my hooves: just good ol' magic.

Author's Note

That book was surprisingly good.
10/10 would clop to again.