Devin looked at the contract and at the mermaid biting her lip in front of him. This was all kind of intimidating. Selling his soul to become a fish person? That was like something in an old story. And yet, what did he actually need a soul for, exactly? So televangelists could tell him that his was going to hell because he didn't send them money? So he could look forward to some afterlife that may or may not even exist? So he could supposedly get reincarnated as something someday, maybe even a cockroach for all he knew? Devin wasn't sure he even believed in souls! He did believe the mermaid when she told him he could live for 300 years if he joined her. After all, if it's turned out that merpeople are real, why not believe the rest of the myth, including their longevity and their ability to transform humans? Surely a few hundred years is as much immortality as anyone needed, and Devin loves the ocean so why not spend it there?
Devin, rubbing his chin and fidgeting in his seat on the fishing boat. A suspenseful theme from an old game show was playing in his head. When the song was almost over, he went ahead and put the quill to the paper, gasping as a surge of energy bubbled up in his throat before creating an intense heat and pressure behind his eyes, nose and lips. These forward-facing openings started to glow, to the point that he couldn't see anymore for the blinding light in front of him, and the mermaid watched gleefully as a sort of pure, glowing energy started flowing forward out of Devin's orifices and coalesced into a glowing figure in front of him.
Devin felt unbelievably strange as his life flashed before him in his shining field of vision. Then, not just this life but previous lives started to unfurl before him: a king, a peasant, a cockroach, a mule, a shaman, a nun, a caveman, a sabertoothed tiger, and even a shark. By the time it was over and the light was starting to fade, Devin noticed that he felt empty inside and that he was having trouble remembering some details about who he was and why exactly he chose to come here. Devin also found himself face-to-face with himself: in front of him, he could see a perfect copy of himself staring into his eyes, a kind of doppelganger as it were.
"Oh, no, what have I done?" asked the doppelganger, looking horrified. "I don't belong out here, I belong... in there," it said, shivering as though it were cold, staring at Devin's body with desire. Devin (or, rather, the soulless husk of Devin) felt a strong, familiar presence emanating from the figure in front of him, a presence that could fill the void inside him if he could just get it back. Soul and body reached for each other, arms passing through each other, desperate to reunite but it was too late. What was left of Devin noticed that the doppelganger in front of him no longer had legs and was starting to become transparent, almost like a ghost starting to vanish. Devin's body watched his soul's fading face twist in agony and heard a faint scream as the uncanny phantom disintegrated into a fine mist which swirled into a kind of nothingness, presuming going wherever it is that souls go when they can no longer stay in this human world. The Devin that was left quivered in terror at what he had just witnessed.
"Huh, so that's what a soul looks like. I've never really seen one up close," said the mermaid as though she were talking about a zoo animal.
Devin looked at her and was about to try to say something when a great ocean wave splashed him, pouring its foam into his mouth and nose, partially filling some fraction of the void inside him. Devin calmed down a little bit looking at the mermaid, marveling at her beauty, then envying it, then burning with lust for the enticing fish woman in front of him. His legs snapped together, fusing and growing out as scales started spreading down their combined length. He was impressed to watch his own feet stretch out into long fins, growing more of the mer scales that were driving him wild. The mermaid giggled and slunk back beneath the surface, and Devin dived in after her to give chase. He pumped his strong merman tail behind him, swallowing in more of the sea foam as he went under. It was merfolk mating season, after all, and it was time for him to have his mate.