Assassin King In Another World

Well, the almighty isekai happens to bring 5 heroes to save the world and beat the almighty demon king but wait there is a 6th hero or not a hero in fact the most infamous assassin in the history of killers comes in and challenges the hero, his companions, and the kingdom that he can kill the demon king much faster by himself than all of them combined. The entire palace laughed at his remarks but wait demon king dies 10 hours later, the demon race has promised to never attack again a month later, and finally, after a year practically all kingdoms and races are begging for an alliance with the Human kingdom just what happened?