Demon gates

The world was suddenly greeted with countless portals opening up all over the world releasing demons that killed half of the human population. With the tragedy that the portals brought they also brought a weapon called aura that unlock hidden human potential. Every year these gates open bring in the wave of demons but with each passing wave we get better at fighting. A warrior that has faced the unspeakable returns back to his world after 7000 years but only 7 years have passed in his world. He has lost all will to fight. Now he has to do something he is not good at nor is he interested i.e. teaching the new generation of fighters. But everyone learns a lesson along the way as he tries to hide his true power while unlocking the real potential of his students.