I originally posted this as a joke RPG concept on a forum I run, but I do find the actual concept intriguing enough I started a story based on it. The concept is a fusion of Sex Magic, post-apocalypse dystopia, and superhero adventure.
The story is set fifteen years after “Eromancy”, this world’s version of Sex Magic, appeared on Earth. Eromancy produces super powers in women, fueled by sex. However, in most women, with most men, the power is still quite weak. For example, she might gain slightly sped up healing, a minor boost of strength, the ability to float, or the ability to vaguely sense surface emotions. One round of sex will usually grant a woman such a power for about a single day, less if they make extensive use of the abilities. Each woman also tends to only gain a single, specific power.
As for the men, they do not gain powers of their own; they instead provide the energy that women use for their abilities. Upon climax during sex, a man unloads mystical energy into a woman’s body, charging her abilities. Most men can only charge a woman with a small amount of energy, resulting in the single-day charge and low-end powers.
But, of course, there are always exceptions. There are men who can dump copious amounts of energy into a woman in one shot, giving them a charge lasting for days. And there are women who gain much, much stronger powers, and even a variety of them at once, from their partners. Put the two together, and you’ve got a woman who can punch through buildings, flash freeze rivers, or regenerate lost limbs, and a man who can grant her such powers for days at a time.
The small handful of men who can produce such power are nicknamed “Founts” and are highly prized by women in society. The small handful of women who can obtain such strong abilities are nicknamed “Loches,” and are likewise prominent publics figures, usually using their powers in heroic manners.
It would have been interesting to see where society went from this point. But unfortunately, Eromancy was not the only supernatural force that visited this alternate Earth. Five years after the sudden arrival of Eromany came the Terror Wave, a mysterious darkness that blanketed the Earth for three days. When it finally faded, the dead began to walk, attacking the living. Monsters made of raw elemental matter began to tear apart cities. Wild animals and plants mutated into hideous abominations that slaughtered anyone who tried to escape into the countryside and wilderness.
Attacked from literally all sides, humanity was forced to retreat into walled off city-states and small, tightly defended countries, losing technology, territories, and numbers until only about one billion people remained. Some believed Eromany could have been responsible for the Wave’s arrival, but no one knows for sure. It cannot be denied that it did help in keeping some of humanity alive, and to this day, the Fount-Loche Corps remains a primary defense against the creatures that dwell outside the countries’ walls, as well as helping supplement scarce resources, and even maintaining peace within their borders.
The story I started writing takes place ten years after the appearance of the Terror Wave, and hence fifteen years after Eromancy appeared, and features a young woman whose powers shift unpredictably after each round of sex, making her a walking gamble as an asset. I won’t say more for now, as I don’t want to spoil it if I ever do manage to make full story of this.
Update: I was able to incorporate this as a sub-setting for the Dellissa's World concept, although as with the SMW, that version completely runs counter to the main setting.