Fate/stay night, dark!Sakura/Rin, making up for lost time
Rin felt the ribbons of darkness slither all over her body. It was ripping off her clothes and caressing her naked skin.
"You've been a terrible sister, haven't you?" Sakura said in a mocking tone. "Leaving me alone to face those monsters by myself. You're going to have to make up for all that time I suffered, you know." She began fondling Rin's breasts, grinning when Rin let out a soft yelp. Rin then felt her sister's fingers rub her clit, and couldn't help from letting out a moan.
The more Sakura stroked her, the less Rin bothered to resist. She knew she was falling into the darkness that had overtaken her sister, and she knew that if she didn't do anything to stop it, the darkness would overtake her too.
But those fingers felt so good, too good, in fact, and it was already too late. Rin let herself go, and dropped into the abyss. She screamed out in pleasure and desire as she climaxed, and then her world turned black.
"Now we'll be together forever," Sakura whispered in her sister's ear once the ribbons of darkness clad Rin in the same slinky black dress as herself.
Rin smiled. She was now part of the darkness, and there would be plenty of time to show her sister just how much she missed her.