Ether

Ether is a substance unique to this world that was discovered by Zodias while he was studying the ancient Ryuujin Tribe Artefacts. It is a substance that is given off by this system's sun and vaporizes into the sea. It has an entirely different nature, making it undetectable by the science and technology from Earth. With the use of artefacts, this new energy source could be used to cause various physical reactions, known as Magic.

It is most likely over use of this natural resource that lead to the decline of the Ryuujin Tribe. And with the high consumption rate the humanity's Airships use it, there has already become an issue of depletion world wide.