In short, your world is get invaded by another world.

They already conquered most of the earth with their technology, while your side is an Fantasy world where's lack of technology but have magic instead. When you're fighting against human from other world, an laser fall from the sky and make an deep hole and you fall to the Void. When you wake up, you find yourself in a Giant cave with full of chain, and in the center you see an girl that chained. You also see two Legendary sword that Hero and Demon queen use long ago.