New life: Savior from another world

It was a peaceful life for humanity. Protected by the order of valiant knights they could be sure the randomly appearing gates to other worlds will be closed, and if any monsters slip through, they will be immediately caught. But on that day everything went wrong. The portal gates wouldn't seal up. The guest from another world wouldn't obey. The young knight was suspected to be self-proclaimed, which led to his imprisonment in a monster detention prison. His seemingly imminent death led to a new beginning. That day is carved into humanity history as future-changing. The day when the portal gates were forced open.