Eos is the name of the world where all of the Places & Locations in Final Fantasy XV can be found.
According to myth, Eos was created by a mysterious race of beings from a higher plain with the Astrals playing a large part in the process. The mysterious beings retreated from the world, while the Astrals would remain to be recognized as a race of their own, with their own language and agendas, and a wish to protect the natural world.
In ancient times, an ancient King of Lucis was gifted by the world's gods with the Crystal, a source of great magical power. In addition to this, the Ring of the Lucii was gifted to the Caelum family, the Lucian royal line, to wield the Crystal's divine power.