A distraught old chess champion's true love dies and in his fit of despair, he summons a demon.
A succubus takes the contract, expecting an easy mark.
Once summoned, she realizes the error—his one true love was playing chess with his old best friend and rival—and she can't return without fulfilling the contract. He begins to teach her chess and, after a time, she begins to teach him to enjoy life again.
One day, many years later, she beats him in a game and vanishes in a puff of perfume and brimstone—her contract fulfilled.
They both spend the rest of their lives trying to resummon the love they didn't know they had.
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Thanks to @jankhambrams for their inspiration to write this.