This is another partially-formed fantasy I've had for a while. It's about a girl known only as "Nil", whose Mistress keeps her perpetually within a whisker of climax while forever denying her satisfaction.
Nil awakes, shivering and trembling, next to her Mistress in bed. The night before, Mistress had spent many hours making love to her, teasing her to the brink of madness. Her training over the years had helped her cope for a long time, but her endurance was finite. She eventually had to beg her Mistress to slow down to allow her to cool off. But Mistress knew her responses too well, forcing Nil to soar ever higher, so dangerously close to her forbidden release that she was sobbing. Eventually, her nervous system had simply overloaded and she'd fainted, falling into an exhausted slumber.