The Ruined World is what remained after the world was split in two and the other world which is called The Living World was created at the same time. However, all the humans in the The Ruin World were erased and only Tokyo remained there because everything else beyond that had begun to collapse.
However, the war didn't end there because the magicians made a pact to continue it but only in The Ruined World and the winner will claim the True Power which resides in The Living World. And to ensure that the magicians could no longer do battle in The Living World, the wise man, a magician of the highest level cast the Gift spell upon it. The Gift is a type of magic that prevents magicians from battling one another in the human world. However, using normal magic is fine but a magician may not use Ancestral Magic to attack another magician.
Today The Ruined World is also the home world of Magicians, while The Living World is home to the humans who doesn't have magic.