It's a world that includes fire, water, air, earth, shadow and light users. This world has at least one archipelago with a king, that's died in the previous squashing of rebellion, his descendant lives known only to few. At the beginning of the series a girl is on a fire-owned submarine vessel that gets attacked by Shadow rebels. The Shadow rebels have at least one base in the archipelago and train people there. I'm not totally sure, but I believe there's also an island that has a lot to do with trade. Also, the fire... clergy, I guess? Has absorbed the light part, so basically there are only 5 separate elements remaining.
Some info I'm not quite 100% sure about, but I think the king of the archipelago might also be called The or a Pharao
Unfortunately this is pretty much all I remember.