Congratulations, you’ve triggered an unskipable cutscene of me infodumping about the Passione and Diavolo/Doppio aspect of this AU, you cannot escape me-
SO BASICALLY shit starts way at the beginning with Diavolo and Doppio. Not fully sure about the dad situation, but their mom was human. Unfortunately, she still ends up getting arrested for Something Or Other, and a young Diavolo and infant Doppio end up getting put into the “care” of the priest guy
now, unfortunately for them, Diavolo has a couple of Mer traits that are starting to poke up, and well…… yeah, no shade to religion as a whole, but you’ve gotta admit, the strict “traditional” types really don’t take this kinda shit well. So Diavolo spends most of his child and young adulthood locked in a basement thing that was connect to but still locked away from the ocean. Like, there’s a tunnel leading outside to the open water but it’s barred off so only water and some fish can get through
Now, the contact between these two was incredibly limited, however the priest guy at least had enough sense to know “okay, letting the monster in the basement actually go insane is a Very Not Good Idea, so let’s see if social interaction can keep him docile” and it both…… does and doesn’t work as intended. On the plus side, for a good chunk of years, Diavolo does just that. On the downside, with every year that goes by, with every year Doppio brings down books and stories from the outside world, with every year he grows larger and the cave grows smaller…….. his hunger for the outside world grows ravenous
So he breaks out via the tunnel and with Doppio’s help. After all, that’s his older brother! He’s sure as hell going to help him out with this!!! And honestly? For the next couple weeks, Diavolo is content with this. He goes out basically every night, usually with Doppio but sometimes without, just to experience what the world has to offer. It's during one of these nights he ends up meeting Donatella which then leads to Trish, but he's completely unaware of her since Donatella was a tourist and not a local
unfortunately, things aren't all perfect. See, Diavolo gets a bit....... overzealous in his newfound freedom, and kinda over does it. That combined with his extreme lack of social skills means that he doesn't take it very kindly to people who disrespect him, especially if some of said people pick up on his mer biology. So he's unfortunately pretty prone to starting fights, most of the time which he leaves no survivors. He hides all the bodies in his tunnel and honestly isn't bothered by it too much. If anything, he's actually kinda proud of it
unfortunately, this backfires when the priest finds out what's up, connects the dots as to all the murders and disappearances, and reacts in a very reasonable, logical way
yeah, so he starts a lynch-
Diavolo and Doppio, understandably, start to freak the fuck out and respond in a similarly reasonable, logical way
Doppio starts a shit ton of fires and Diavolo grows to full size to smash the place
come morning, the whole town is nothing but ash and there were no survivors. Diavolo gives Doppio a ride to the neighbouring island and they both decide "okay, let's not do that again, what could be do to make sure we don't get put in a situation like that again?" and decide the safest long term plan for them would be to gain power. Legal power however was a bit too out and open for their tastes, so they decided instead to gain underground power. They start building up their gang, recruiting people and starting their "trade," and they actually draw in quite the crowd. Given how accepting the two were about mers they ended up finding a very untapped market of manpower that was free for the taking
and of course, when they find on of the Arrows this power just increases. The Beetle Arrow (or maybe it's some kind of other animal? maybe an octopus?) is still Super Power and different from the others, and also something they only had temporarily before it was snatched up by Polnareff, but they've got at least one regular arrow which gives humans a bit of a slight power boost. Maybe instead of fully transforming them, it gives them a randomised minor mer-themed ability, ex: slightly boosted strength or can hold their breaths for a long time or more durable skin, etc etc
it's enough that there are a LOT of rumours of Passione's crews, but they're also still obviously very human which insights a lot more cooperation from those under their "rule" out of general fear
I'll admit, I'm not super sure how Diavolo and Doppio learn about Trish's existence, but that whole part of the AU is Diavolo wording the Bucci Gang to pick her up on one island and bring her to another island on the other side of the "Italy" part of the archipelago. Maybe for some Fun Team Bonding because this is in the equivalent of Pirate/Victorian Times For The Aesthetic <3 it takes them several weeks instead of just one. Giorno is a Weird Cabin Boy that Bucciarati dragged back to the ship who gives them all Vibes, but none of them actually suspect him of being a proper mer because they've all gotten used to the Vibes that come with being stabbed by the arrow
