The Swords of Darkness

A cadre of four siblings, three boys and one girl, the eldest being Ichiro, a boy of 13, live without their parents in Serran, the capital of The Spirit Island as their parents are sailors and hardly ever meet them. Everything goes well. But of course,
only as long as god wants it to. On his thirteenth birthday Ichiro receives a letter and a gift apparently from their parents. The letter claims their parents to be dead and the gift being the fabled spirit fruits. There are instead four metallic spheres in the box. Definitely not something a human can digest. None of them believes the letter, but when Ayami, the youngest of the four, takes a bite from one of those four supposed-to-be-fruits, and Ichiro follows, they are gifted with supernatural powers.
Following the instructions of the letter, they then start training with the newly-gifted powers and on Ichiros fourteenth birthday, set off on a journey to revive their parents by defeating the evil soul, the one that has managed to keep the Spirit God a captive.
To become worthy opponents of the evil soul, they have to go through twelve forts at each of which they have to defeat a guardian, most of who arent even humans, and thus obtain more spirit fruits, each one making them stronger. The guardian of the very first fort is an immortal genius and renders the siblings powerless, literally, though a machine called the disc of power.
Will they fight through it all? Alone? Definitely not. With some help? Who knows.