The following story has futanari.
This chapter takes place in a version of Heaven's Feel's true end if Saber survived.
Rin Tohsaka had a bit of a problem right now. Actually she had been having a problem for the last couple of days. While she was at the Clock Tower she had been subject to a bit of a strange accident. There was some alchemist raving about an elixir he had created that would revolutionize medicine. Unfortunately in his raving he bumped into Rin and spilled it all over her. It didn't have any immediate effects so she thought she was fine and decided to leave him alone. That is of course after firing a fair number of Gandrs at him on principle however the next day, the day before she flew back home, she noticed a bit of a conundrum when she awoke. She grew a dick. She went about finding the alchemist and demanding that he reverse it. He informed her that the only way to reverse it would be a tantric (sexual) ritual between her and two other people. He offered to join her to which she responded by using one of her gems to curse him so that there would be the sensation of getting stabbed in the balls with a hot knife for a week. The gem in question had cost her a couple hundred thousand yen. It was worth it.
Having flown home and having just eaten lunch at Shirou's she was faced with a problem. Even after trying a couple of different cures none worked and so she was considering who do the ritual with. She immediately assumed Saber would help, the girl was too kind to say no, but she couldn't decide on the third participant. She wasn't sure if Shirou's puppet body would cause problems with the ritual. Of course Sakura was out the question given that they are sisters. Even if she did have a nice rack and an ass to die for. No! Bad older sister thoughts! That left Rider, who was certainly a nice sight to behold with her long flowing lilac locks and long legs. Not to mention her hypnotically beautiful eyes and voice. So it was decided. She would ask Saber for her help and hope that Rider would agree to help as well.
It took a bit of fumbling around in terms of conversation before she actually got the gumption to ask Saber.
"Don't worry Rin, I am sympathetic to your plight and would be more than willing to help."
"Thank you so much… but why exactly are you especially sympathetic?" The magus really shouldn't have looked a gift horse in the mouth, but dammit she was curious. "It involves a bout of mischief started by troublemaking old man. I would rather not discuss it in detail."
Then came the task of asking for Rider's help. Rin knew the Servant got along with her well enough but didn't know if she would accept such a request. Nonetheless she had to ask here and so ask she did. There was only one condition.
"After I help you with this ritual you have to be my servant for a week. That includes any requests to dress up like a maid or nurse or sweet, sweet Ayako-chan~" All things considered Medusa could have made much worse demands. If a week of servitude was what it took then Rin felt that it was worth it. She then led them to her bedroom after making sure that Shirou and her sister had left.
Soon enough Rin started to give Artoria gentle kisses before increasing their intensity. Meanwhile Rider began grinding up against Rins backside while nibbling at her earlobes. Rin started to unbutton Saber's blouse, pausing a bit for Rider to remove both her own turtleneck and Rin's. Saber was without a bra, Rin supposed she really didn't need them given her small breasts. Rin marveled at Rider generous bust, sweet ass, and, long beautiful legs. She also marveled at Saber who was beautiful in a more regal sense. Soon enough they were on the floor with Rider on top of Rin and Saber to the side, all of them bereft of clothing with the exception of Rin's thigh high stockings and Rider's glasses. Rider positioned her clit in front of Rin's mouth while she stuck the magus' now hard cock between her generous bosom. Rin began lapping at the woman's folds and savored the taste as Rider started pumping the cock between her tits. Rin wrapped her stocking clad legs around Rider's head as the Servant licked at the tip of cock as she picked up the pace of her pumping. Saber meanwhile began to touch herself as she got on the floor and began licking at Rin's anus. Rin shuddered at the pleasure from the new sensations she was feeling, both from her new appendage and from her previously untouched hole. Rider's tits were hot and beginning to get sweaty while Saber's tongue felt odd but pleasurable in her virgin anus. Soon enough Rin cock spurted a generous load onto Rider's face as the lilac locked lady came herself from Rin's ministrations. Rin and Saber then licked the cum off of Rider's face and glasses and savored the taste.
"Why the hell isn't this thing going away?" asked the Magus as she already began to harden once more. Replied the King of Knights, "We'll just have to keep going then." After which she began to go down on Rin's kneeling form. She spat on the cock before giving it a good jerk off with her hands while she suckled on the girl's sack. While this was going on Rider gave Rin an intimate tongue filled kiss as she caressed the magus' right breast while twisting the right's nipple. After a while Saber the whole cock into her mouth while fingering the twin-tailed girls ass. Rin couldn't help but cry out as she came inside the Servant's mouth while being caressed by Rider. Gagging loudly from the sheer volume of cum, she took it all in her mouth before quickly standing up and kissing Rider on the lips, sharing the seed between their mouths. They both swallowed the cum and then ended the kiss between them as they looked at Rin with lust filled gazes, a thin trail of cum still hung between their lips.
After pushing Rin to the ground Rider set herself down on the cock while facing away from Rin, giving the magus a nice view of the Servant's ass as she bounced up and down on top of her. Saber planted her folds onto Rin's mouth as she sat on her face. Rin suckled and licked away at the petite Servant's clit and folds. Rin gave in to her urge to slap Riders bountiful ass as the Servant increased her bouncing pace, eliciting a lustful gasp from Rider's lips. Rider though to herself that the cock felt better than most men's probably did as she shuddered at the sensations of the pulsating cock inside of her. Rin shot enough hot seed into Rider that the Servant certainly be impregnated if the seed was not artificial in nature. But Saber still need her fix. Saber got on her hands and knees while wiggling her hips practically begging to be ravaged. Rin plunged straight into to Servants pussy. Saber was I different experience than Rider, Rin thought to herself. Even if Saber's body didn't give off the raw sex appeal that Rider did her pussy was even tighter, barely able to hold Rin's cock. While she pounded away at saber Rin's ass was being licked by Rider who was fingering herself as well. There was no way Rin could last long inside of her while getting her ass licked and soon enough she sprayed her seed into Saber as she felt the petite girl climax, her inner walls becoming even tighter than before. Rider also fingered herself to climax. After that Rider made sure to lick the cock clean and soon after it finally began to fade away and the three women rested their tired bodies.
Rider spoke between panting breaths, "Rin you should look into getting more of whatever grew that so we can enjoy it on occasion." Added Saber, "I concur with Rider on this." Rin thought it over in her head, considering the sensations that she had never experienced before she received the new appendage. Of course with how much she had hurt the alchemist that made it she was certain that he would charge a truly exorbitant price for anymore of the elixir. She decided it was worth it.
Author's Note: I would like to thank Soulvanguard for helping me improve my chapters even if I still have a lot of improving to do. As always please review and leave and idea for pairings and sex acts/kinks.