I have this Marathon mer-AU that was originally a lower-key slice-of-life thing; that got adjusted when I returned to it. Some more text here: https://general-radix.dreamwidth.org/16996.html Ctrl+v from the original dA submission:

"Got inspired to return to the mer AU; previously it was more of a slice-of-life thing, but after all the vamp AU stuff, I wanted to do something different with it, and this idea felt like a good way to start.

Previously, Vince was a battleroid like in regular Inm.; for version 2 of this story (such that it is at the moment), he's something a little more thematically-relevant: a sort of fish-hybrid mutant, courtesy of Bernhard Strauss and his fantastic ancient artifact of evil. These mutants, called nixes/nixies, were somewhat common in Earth's distant past and tended to get along with neither merfolk nor humans; in the present day they're all but completely gone, but Strauss figured out how to bring them back. Unfortunately.

Vince is the tenth victim of the nix-creating process, and he would've been mutated even further if Durandal hadn't interrupted it; after they escape Strauss's clutches, the two travel to Durandal's old home near a seaside village out of necessity (Strauss knows where Vince lives and managed to give the police the slip), whereupon Vince gets to deal with people who have no idea how to regard nixes. The merfolk are more understanding, but there's a lot of things Vince can't do with them, like "hold conversations underwater"; he's basically stuck between two worlds, though Durandal's determined to help him with it.

Ability-wise, Vince can breathe underwater as long as his gills aren't covered; becoming a nix may or may not confer water-manipulation powers (to parallel vamp AU!Vince's blood arts). His spines can comfortably lay flat against his body, though because they can still catch on things, he doesn't bother with wetsuits and just swims shirtless most of the time. Also, his sex drive occasionally acts up to levels even Durandal, who was pretty active even before meeting Vince, finds exhausting; Vince is slightly less conflicted on this issue than the others, if only because he was going to end up making love to Durandal often anyway. :U"