Trigger

The world was suddenly invaded by invaders from another world, and most of the world was destroyed by these monsters, leaving behind a wasteland. The only safe place is now the Star Kingdom. At the moment of the invasion, 7 people appeared so strong that they were able to easily destroy the invaders, but it was too late for the rest of the world. They were people from the other world, but they didn't have too much time, so they passed on their fight methods and explained what was going on. Now, the Kingdom gets stronger as these methods are passed on into soldiers trained by the kingdom. With a school specifically for this reason. The son of the Commander of the Star Kingdom was trained since the baby to be the strongest and is the hope to recover the land lost to the invaders. He has to fight against strong invaders that keep coming out of portals and find the truth about the other world.