The Shadow over Innsmouth
The backstory of "The Shadow over Innsmouth" involves a bargain between Deep Ones and humans, in which the aquatic species provides plentiful fishing and gold in the form of strangely formed jewelry. In return, the land-dwellers give human sacrifices and a promise of "mixing"—the mating of humans with Deep Ones. Although the Deep One hybrid offspring are born with the appearance of a normal human being, the individual will eventually transform into a Deep One, gaining immortality—by default—only when the transformation is complete.