There's a bit of a twist I've got in mind that I don't want to give away completely, but it's less magic dying out and more someone wiping out competition. >.>
It sounds good, and would make a good foundation for a series. Maybe it's the Shannara series I'm thinking of, like Mith said. I read part of the first book, but could never finish it. I don't recall it enough to know if that's what I'm thinking of or not.
The whole inspiration for this goes back to a song from one of my favorite bands (Flogging Molly) titled, "Seven Deadly Pirates, Seven Deadly Sins." The title planted this idea at the back of my brain of this crew of pirates who actually embodied the sins. Each would be prone to indulging in their sin and can inspire it in others, along with a few other abilities. I've actually only seen a handful of Fullmetal Alchemist episodes so I might need to research that a bit to avoid copying it too much.
I could see it being good. The FMA series had characters that were human-like, but were associated with the seven deadly sins, including some of their abilities based on their traits. But I think if you kept your characters more human, with human interaction, it'd be significantly different. They were mostly loners, from what I recall. It's been a while.
I've got some ideas based on your synopsis, if you care to read them. I don't mean to step on your ideas by any means, and if I'm overstepping, feel free to let me know or ignore me.
- I'm picturing something like a seven book series, with each book focusing on a different of the pirate Sins. Your 1st or 3rd narration would follow the heroes, but maybe even each book has story views from the pirate that book is focusing on. Maybe some books are wins for the heroes, and some books are falures. Maybe some of the Sins are recaptured by the big finale, but others are still at large, and close to succeeding.
- As you said, seven pirates could come across the Sins, and embody their nature. I think it'd be interesting if they were all part of a single crew, but after finding the Sins, they all split off on their own, becoming captains of their own ships. They could have some interaction and cooperation as needed, but might also have some competition between them. Maybe they even have their own sea (Seven sins, seven seas) that they're operating from, to work their bad works.