The Immortal Demon King and the Impaired Hero

The demon king was summoned from Japan to fight off the hero. After a long grueling fight he finally won against the hero. But something strange happened. After he had defeated but not killed the hero the holy sword floated up and stabbed the hero in the chest and then immediately disappeared. And this isn’t the only time it happened. Every hero after that that wielded the holy sword was killed by it immediately after the hero lost to the demon king.