The planet just survived a world class catastrophe. Populations were at an all time low.
That's when the almighty one sent a divine being, with a special power at his disposal.
The power called Erotic Reality, or ER.
The power to bring out the deepest sexual desire and fetish of a mortal, and make it a reality.
Not only that, any girl that sleep with the divine fox will be guaranteed pregnancy on her next time with a male. While a male that drink his milk will also get guaranteed child the next time he lay with a female.
His task is to make re-population easy. And he performed his duty admirably for hundreds and thousands of year. And he was then deified as a god of sexual favor and fertility.
But, a demon lord's own whim caused his death. his soul was corrupted to make what was eventually known as Ixy the Incubus. While the body returned to the cauldron of life and become the mortal fox known as Foxy.
Ixy is ageless like the divine, while Foxy keep reincarnating throughout the passage of time.
As for the power itself, Ixy managed to harness a watered down version of ER in his illusion magic. While Foxy become the beacon of ER, now owned by entities that used it for fun.