Celestia knelt in the sunlight streaming through the window, ignoring her visitor. It was funny, really. It only took a few weeks after her return for Nightmare to realize that daylight was necessary.

She knew full well what the demon that had possessed her sister had come to do, or at least she thought she knew.

"Which of my orifices have you come to invade now? My pussy, my ass, or my mouth?" She asked, not even looking.

Nightmare smirked, ready to catch Celestia off guard.

"I came to bring you a gift."

She rolled Twilight's severed head towards her once sister.

As she looked down, she gasped, tears forming in her eyes.

"Where's the rest of her?" She asked.

Nightmare patted her swollen balls.

Celestia broke into full tears.

Nightmare, pleased by this, spoke. "To answer your earlier question, I've come for your pussy."

Celestia looked up. "You see, I'm so full of spunk," she leaned back, showing off her watermelon sized balls "and it seemed such a shame to waste it on masturbation. Then it struck me: Why not create an heir!"

Celestia blushed. "You want to impregnate me!"

"Yes. It's the only way to be sure my heir is an Alicorn."

Nightmare looked Tia over. Her large bust and wide hips spoke of fertility. Her pussy was already moist, showing that she was turned on by the thought of being forced into bearing her sister's baby, even though she would never admit it. Even Celestia's futa cock was slowly rising, a fact she was trying to hide.

Nightmare, feeling her own erection stiffening, approached her sister from behind, her own cock resting on Tia's back, and reached around. Grasping Celestia's rod with both hands, she began to jerk her sister off.

"Give in sister, you know you want me in you."

"Never!"

"Very well. If you will not give me what I seek, I will take it!"

She stepped backwards, allowing her dick to hang freely, and guided it in with her hands.

"Tight as ever, dear sister."

Celestia gave no answer, save to cross her arms.

As she began to fuck her sister, she resumed jerking her off. It took several minutes, but she finally got her sister at full mast.

She looked down at Twilight's head. "I think I know how to make this a little more interesting..."

She picked up the severed head and looked at it. "You don't mind, do you?"

She opened the mouth and slowly lowered it over Celestia's cock.

"Come on, didn't you ever want that little slut to suck you off? I've seen what she wore around here, and I can tell you: she was trying hard to seduce you. I wonder if she ever knew of your 'endowment'."

With that, she slammed the head down Celestia's shaft, smearing blood on her cock where it exited Twi's throat.

Celestia, despite her reservations, began to moan. She had often gotten off on thinking of jamming her cock down Twilight's throat. She had to admit, the warmth and tightness of her throat was even better than in her fantasies, even though she was dead.

"No, I must resist!" She thought. "If I let her pheromones intoxicate me, I won't be able to stop her from... "

Nightmare increased the intensity of her thrusts, slamming against her sister's cervix.

"Can't let her... Make me cum... Can't... Let... Her ... Break... Me!!!..."

Alas, her resistance was futile.

Celestia cried out, cumming from both her cock and pussy. She sprayed the floor with cum, slicking it.

Nightmare slipped, falling on her back. Celestia stood over her, a triumphant look in her eyes.

In stead of trying to get away, however, she slowly lowered herself on top of Nightmare's cock.

"Let's do this at my pace now. How does that sound."

Nightmare smiled. The corruption was taking hold.

Celestia lowered herself onto Nightmare's mighty girth moaning at the fullness. Sure, this was wrong. It was incest, after all. But it just felt so good, so right.

She picked up the pace, adding a slight rocking motion in her hips. Nightmare moaned. She never knew how good her sister was.

"Ready to fill me up!"

"Yeah!!!"

Celestia dropped down, impaling herself and leaving Nightmare's flare pressing against her cervix.

It was Nightmare's turn to cry out: "I'm cumming!!!"

Celestia let out a long, slow moan as a thick, powerful jet of cum hit the back of her uterus. Her womb began to swell, filling with Nightmare's fertile cum. Celestia's belly ballooned to almost cartoonish proportions. She dismounted and laid back, angling Nightmare's cock to shower her in cum as more flooded out of her vagina.

She passed out.

An hour later

Celestia awoke to a wonderful sensation on her cock.

"What the hay?"

She sat up, and saw Nightmare giving her a sloppy blowjob.

Nightmare savored the taste of blood and cum on her sister's dick. Taking a deep breath, she pushed down, taking it as deep as she could.

Celestia moaned "Yes, my Queen! Take me deep."

Nightmare held it as long as she could. Once she was about to pass out, she lifted back up. She left in just the tip.

She played around with it with her tongue for a few minutes, then she deepthroated again.

She repeated this process for a few minutes, drawing more than a few moans from Celestia.

Finally, she felt Celestia's cock spasm in her throat, and Tia, putting her hands on Nightmare's head, forced her full length down. Nightmare felt her stomach swell with Celestia's cum. Nightmare's lungs were screaming for air by the time Tia let her up. The last streams came up in her mouth, swelling her cheeks and sending cum through her nose.

Nightmare sat back, wiping the cum from under her nose, swallowing what was in her mouth.

Celestia smiled, watching Nightmare lick her fingers.

Nightmare got up. "That was fun."

Celestia nodded. "I don't know why I resisted. That felt good!"

Nightmare smiled. "Let's do this again some time."

Celestia put her hand on her still distended belly. "Do you think the sperm took."

Nightmare kissed her. "If it didn't, we get to do this all again."

Celestia giggled, "that doesn't sound too bad..."

Meanwhile, in a different equestria...

"It was just a dream, Twilight, nothing more." Celestia said, trying to reassure her former student.

"Well, okay."

Twilight looked up at the clock.

"Shoot, I'd better go. I promised to see my parents for lunch!"

"Then go!"

Twilight hurried out of the room. As soon as the door closed, Luna stepped out of the closet and sat next to her sister.

"Did you get a chance to scan her?" asked Celestia.

"Yes."

"And..."

Luna sighed. "I can't be completely certain, but... I think she did travel between worlds in her dreams last night."

"But that would mean that that awful world is... real! But how would this happen?"

"Maybe she's meant to help that world."

"Hopefully, dear sister..."

Author's Note

And so this ends. But I'm thinking about doing a sequel with the remainder of the mane 6. What do you think?