Prologue

An unknown and hostile army had invaded New York City. With their mythical creatures and warriors clad in full steel armors, they mercilessly cut down the city and every person they saw within sight. With the U.S. military armed forces arriving, the invaders had been repel but such victory came at a high cost. A new mission was established afterward: journey through the portal and find the man responsible for committing such atrocities upon their land, either through brute force or peaceful option.
P/S: Took some inspiration and concept from GATE anime and manga