i captured all 32 cities and had to wander a bit to find a weapon capable of dealing 500+ damage. The weapon you are looking for is enchanted and has the name of "Mysterious [any weapon name] of Death." I completed mine with the mysterious cheap katana of death (1-7 dmg) but the enchantment adds: hit chance 100%, crit rate 90%, and dmg 0-1000! also lowers def and dodge to zero and decreases hp by 80%.
I found the weapon right next to Arretium. gl!
 "Feudalism"){kind=icon}
 "Arkandian Crusade"){kind=icon}
 "Champions of chaos"){kind=icon}
 "Feudalism II"){kind=icon}