Prolouge

Grandma Kuzanagi once told a tale to Iron and Rana; Iron and Rana were 4 and 7-years-old at the time. They were lying together in bed, listening to their grandmother's tale.

The two children recall this tale years later:

[Eons passed, this world continues to evolve."

"Monsters will rise, new legend will arise.

"Soon, this world will become, something unknown...]

The two children, Iron and Rana, didn't recall the rest of the tale.

Safe to say, it's still a mystery...