Inspired by the amalgamation of prompts from a bunch of anons.
 
>You are Anon, and currently you are inside a dark brooding chamber filled with weird gothic pre-modern decor being prepped for...
>Really what are you being prepped for exactly?
>"Don't question it, boy, now strip." Your father commands.
"I'm sorry what?"
>Your father looks at you from under his hood.
>"Don't be such a faggot my son, I'm your dad. Or would you rather have your mother be here inste- wait a minute... I knew this day would come the moment I first gazed upon you! I won't let you have my beautiful wife!"
>He points a finger at you.
>Ah, good old dad, what a weirdo.
>But yeah, he has a point, nothing wrong with being in the nude with your old man right?
>Right.
>Quickly you don your birthday suit.
"Now what?"
>Without a word, your father picks up a bowl mixed with some dark red liquid inside and begins painting weird symbols on your body.
>Wait a minute...
>These are runes, mighty Old One runes.
>"I'm so proud of you son, to be picked to take part in such a glorious ceremony for our Dark Mother...
>Ah, that explains the people out in the antechamber in ceremonial dress chanting and prancing about.
"What is this anyway?" You gesture to the markings drying on your body.
>"My blood."
>Wut?
>You're at a loss by how nonchalant and quick his answer was.
>The look on your face must have been very telling.
>"Oh relax, it could be so much worse you know? Did you know certain indigenous African tribes have their boys drink the semen of the men for their coming-of-age rite? These tribes are small my son..." He trails off.
>What is he impl- Oh sweet 'fhalma.
"And where does Sunset fit in all of this? Where did mom take her?"
>"To the chamber opposite of ours of course, just like you she's being prepared for her ceremony."
"Hers? Wasn't this my coming of age?"
>Your father clicks his tongue from behind you, the brush swiftly doing its job at your back.
 
>"You bring shame to our line Anon, didn't you pay attention when your mother and I were explaining to you two the intricacies of the rite?"
>The only thing you remember were your parents gushing and singing praise to Xrphonax while Sunset took notes.
>You kinda let it enter one ear and leave the other.
>"Both your rites overlap; there can't be one without the other..."
"Huh, that sounds a lot like your explanation for why Sunset and I had to take Sex ed together. Or why we had to live together in the guesthouse..."
>"The bond shouldn't be severed for long, you know? Why do you think everyone is always together with their spouse?"
>That IS pretty weird.
>"Anon, why do you think our community always bores fraternal twins?"
"Something in the wate- HEY!"
>Your father pokes you with the back of the brush.
"By the way, where IS your twin sister?"
>Dad looks to you like you grew a second head.
>"How much of our lessons did you skip!? 'Tis one of the gifts of our Dark Mother; we are blessed with our other half at birth, all that is asked of us is that we do not squander our gift!"
"Soo-"
>"My son, how can you be this retarded? Xrphonax's blessing prevents such weakness in our blood! It's your mother!"
>Oh...
>Wait.
>O-Oh?
>YOOOOOOOOOO!?
"You're married to your sister!?"
>"Yes, we've been together as man and woman ever since our coming of age."
>You let that sink in.
>Meanwhile the large double doors open, revealing a man and woman wearing the same cloaks as your father.
>"Incognito are the preparations ready?" The man asks.
>"The moon will be at its zenith soon." The woman finishes.
>"It's finished, we will follow shortly."
>The couple leaves.
>Dad hands you a cloak.
>"Put this on; do not bring further shame to our line with your lack of awareness Anon."
 
>Cloaked with the dark purple treads given to you and seemingly branded in runes written with the blood of your own father, you walk into the large antechamber.
>Large stone pillars hold up the ceiling up above, adorned with ancient paintings of celestial bodies surrounding a sinister looking figure.
>The entire room is lowly lit by torchlight.
>At the junction between your chamber and the one in front of it you meet up with your mother and Sunset.
>Both wearing the exact same cloaks that cling to their bodies yet hide their figures, same as you and your father.
>Your mother is absolutely giddy, grinning ear to ear whereas Sunset's eyes bore into you from behind her hood.
>Her cheeks are a bright red.
>"'fhalma shagg-oth c-vulgtlagln...!" The words echo in the large antechamber, the acoustics of the room giving it an otherworldly effect as cloaked figures chant and clasp their hands, and others beat down on large drums that reverberate around you.
https://youtu.be/ctwZfJWrm3I [Embed]
>You remain quiet as your parents leave to join the rest of the crowd.
>Sunset stands by your side and grabs your hand before pulling you along deeper into the antechamber.
>You don't say anything, what's unfolding before you taking any words you might have away.
>"c-chtenff tharanak gof'nn hupadgh tharanak!" The deep voice continues to bellow upwards.
>Shit, come on Anon, you know this, you've been raised around these people, you studied the language...
>...Our society... brings children born of promise?
>As you walk, the center of the room draws closer.
>The old man speaking, lights a bonfire.
>In doing so, a group of women begin to dance around it, chanting as the fire grows stronger.
>Sunset's grip on your hand strengthens as you reach the center.
 
>After the climb of a short set of stairs you two come to be in front of a large statue, chiseled from rock to the equine-like shape of Xrphonax.
>On this platform, you are surrounded on four sides by braziers, burning a deep blue fire.
>Beneath your feet is a circle carved into the stone, and above your head, up above in the ceiling, is a huge chunk of it missing, letting the pure night sky seep in.
>The full moon already peeking in on this. 
>Sunset turns to face the cult, her grip on your hand causing you to follow.
>You can see the entire place.
>You even manage to pick out your parents.
>The old man, beneath the platform, continues his entranced utterances
>"Relax Anon..." Sunset finally speaks "... You seem really tense."
"I have no idea what we're doing."
>She frowns.
>"Didn't you pay attention to mom and dad?"
>Again with this, you like to daydream, okay?
>She sighs "Just follow my lead."
>The dark ceremony seems to be reaching a crescendo and at that moment Sunset disrobes, showing that, like you, she was wearing nothing underneath.
>Her nubile body is covered in runes as well.
>On her arms.
>On her supple chest.
>Down to her soft stomach.
>Even on her noticeable thighs, hips and legs.
>You just follow her lead and disrobe as well, suddenly feeling incredibly self-conscious at being in the nude and on display to a bunch of other people celebrating it.
>Even Sunset's cheeks show she probably feels the same.
>"Just focus on me, Anon."
>Right, just focus on the naked Sunset in front of yo- is that the showings of a small bush? Oh lawdy...
>As the ceremony climaxes the old man speaks.
>"'fhalma c-sll'ha-og li'hee lloig, nog nnn-c-gof'nn k'yarnak f'-'bthnk, twincest-yar, y'hah!"
>He raises his arms up above his head and assumes the hand gesture of your cult.
>Hands together, fingers out and interlocking with each other.
>The chanting and music stops as everyone in the room repeats the words and does the same gesture.
 
>From the corner of your eye you see Sunset turn towards the large statue and raise her arms in the same gesture, giving you a view of her hairless armpit.
>You assume the same gesture after a delay.
>"'fhalma, shagg-oth, c-sll'ha-og li'hee lloig, nog nnn-c-gof'nn k'yarnak c-'bthnk, twincest-yar, y'hah."
>Sunset's voice is clear and confident.
>Whereas you just don't say anything at the ridiculousness of those words.
>However, through the silence of the entire antechamber you swore you heard the most familiar male groan...
>Just do it.
>You straighten your back and get serious.
"'fhalma, shagg-oth, c-sll'ha-og li'hee lloig, nog nnn-c-gof'nn k'yarnak c-'bthnk, twincest-yar, y'hah."
>Translation: Mother, native of the realm of dreams, we invite you on pain of mind come watch we children share our essence. Time for Twincest, amen.
>After those words leave your mouth, the moon stops right above you, and seeps its light through the large opening in the ceiling and towards the horn of the statue.
>With that the statue's eyes begin to glow and cast a spotlight on the circle you are standing on.
>The quiet gets broken by yet more music and chanting.
>Sunset's body presses against yours, her breasts squeezing against your chest.
>She wraps her arms around you and brings you down slightly for a kiss.
>The kiss is chaste, owing to her lack of experience, but then again, you are on the same level.
>In the back of your mind there's a voice screaming "she's your sister!" but being constantly raised on the idea of always being with her, how this is okay, coupled with your hormones makes you not care.
>You are amazed at how little you don’t care.
>You press Sunset closer to you, feeling her hot branded skin on yours.
>Around you there's a cacophony of instruments and chanting along with some pagan-like dancing.
 
>This is surreal to say the least.
>You gasp as you feel your hardening member get grasped by a soft hand and gently stroked.
>You look at Sunset, a face mixed with some surprise.
>Her half lidded eyes show complacence.
>"I've been waiting for this night for months Anon~" She whispers near your ear, her warm breath causing you to shudder.
>"I'm so glad we get to be together like this..." She plays with one of your dried runes with her free hand while still stroking you with the other.
>"How about we give the Dark Mother Xrphonax a show?"
>Sunset sinks down to her knees...
>And, right in front of everyone, wraps her lips around your cock.
>You inhale at the feeling of her mouth suckling your head.
>You try not to look towards the crowd after you swore you heard a familiar feminine "atta girl!"
>Just pretend they aren't there.
>Pretend your parents aren't watching you as your sister gives you head while she fingers herself.
>Just focus on her, Anon.
>On her glazed eyes as they glance up towards you as she takes more of you deeper into her mouth...
>Goddamn it Sunset.
>You let out a groan and instinctively stroke her head.
>She finally releases your member from the wet confines of her mouth, leaving it slightly throbbing and covered in a sheen of saliva.
>"I think you're all nice and ready..."
>She grabs your arm and tugs it, gesturing for you to get down on her level.
>She turns around on all fours and presents her supple bottom to you.
>She gives you the sultriest over the shoulder look you've ever witnessed.
>Now that you catch a good look at her from this position, you notice her markings seem to go up her thighs and down her backside, but all clearly pointing to one place. 
>Her damp, slightly parted slit.
>You swallow the dryness in your mouth, the ceremony an afterthought as you inch closer.
>You grab a handful of your sister's behind and guide your worshiper of Xrphonax to her entrance.
 
>The music stops momentarily, reminding you that you have an audience.
>Quiet fills the antechamber as you push forward and part Sunset's walls.
>"O-Oh~!" She mewls as you meet resistance.
>You grab hold of her noticeable hips and jam it in the rest of the way.
>"Ah~!" her high pitched yelp echoes throughout the antechamber.
>The ceremony picks back up as you slowly move in and out of the shaking Sunset.
>Some blood drips down her entrance and mixes with the markings on her inner thigh.
>"Anon..." She whispers between breaths.
>The sight of your sister's marked back as you take her from behind becomes your sole focus.
>The way she relaxes her back when you pull out and the way her back clenches when you slide back in...
>Her body showing signs of overheating as sweat slides down her sides...
>You want to see more.
>You pull out and have your twin sister lie on her back.
>You wince slightly at the fact there's nothing between her and the hard rock, but such thoughts are thrown out of the window when she opens her arms to await the return of your embrace.
>An embrace you gladly give.
>Sunset lifts her hips to meet your thrusts as you two reach a rhythm in tandem with the drumming, entranced in your own little world.
>That being her bouncing breasts...
>You lean in and lick through one of her pert nipples.
>Sunset runs her hands through your hair and moans openly at the attention you're giving her.
>"A-Anon~ hmm, don't stop!"
>You nibble lightly on the tender button as you brush your teeth against it.
>Sunset arches her back to give you more access, but that's impossible, you're already practically motorboating those sweater puppies.
>You hook an arm around her back and lift her up into a sitting position.
>She wraps her legs around your waist and grinds into you, your cock head completely at the mercy of her walls without the protection of any skin.
>Each time she brings herself down on you, you can see slight droplets of sweat in the moonlight.
 
>"Ano- Ah~!- Anon I'm almost there!" Sunset moans into your ear.
>Each time she brings herself down she grows more and more desperate.
>She's practically bouncing on your dick.
>To be frank, you have no idea how you've managed to last this long but now with the thought on your lust addled mind you feel pressure build up from your balls to the base of your shaft.
"Oh crapcrapcrap"
>You curse as you feel Sunset tighten around you, seemingly urging you to just let go.
>"ANON!"
>As she yells out your name to the heavens you let out spurt after spurt of alabaster cum deep inside your twin.
>You feel your thighs get soaked as Sunset slightly convulses on your lap.
>Eveything's quiet again, save for the ragged breathing of the both of you.
>You lie on your back while Sunset still straddles you, grinding herself on your slowly deflating member, riding out her own orgasm.
>The moon moves away, the light abandoning the statue and leaving you with only the dark fire of the braziers.
>You did good right?
>You satisfied the Old One right?
>Nobody's losing their minds or anything so you guess you're fine?
>Then again, you have to contemplate the state of mind of the people who'd set this up.
>Sunset leans down on you, her breasts again squishing themselves on your chest, and plants a deep kiss on your lips.
>"We'll be together forever, right Anon?"
>As if you'd ever leave your sister.
>You reach out to the side and grab one of the cloaks to give to Sunset, something your sister appreciates as she quickly dons it on.
>You're still the center of attention for a large group of crazy cult people, your parents included, after all.
>But, to be honest, you don't really mind, your existence and that of your sister is thanks solely to these people.
 
/End.
 
>Meanwhile, IN SPAAACE.
>You shake the hoof coated in your juices, watching the liquid float around slowly.
>You're Nightmare Moon, and you just finished rubbing yourself to another set of depraved human siblings rutting for your entertainment.
>At first it was just a joke, honest.
>Bored out of your mind as you were you just wanted to see how far you could make them go.
>Never in your mind did you imagine them taking to it completely.
>You even went pro bono and spiked their blood with a little bit of your magic to counter act the potential problems they might experience.
>Your esteemed self even went has far as cause them to always breed an opposite sex pair, just to make you appear even more outworldly in their minds.
>It certainly isn't because you find twins to be absolutely lewd or anything.
>It's not like you, the powerful Nightmare Moon, would get off to incest.
>It was a simple ruse!
 
/Alternate End