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.
The demon king was heartbroken upon witnessing this
He tried different ways to save the heroes from their fate.
He tried destroying the sword but that was impossible.
He tried stealing the sword but it always kept disappearing.
He tried telling the heroes the truth about the sword but most won’t believe him.
And those that did believe him were immediately killed by the sword.
The demon king watched this go on for nearly 500 years repeatedly. And with each hero he defeat he gets stronger and stronger to the point where no hero can beat him anymore. He had gotten so powerful that the holy sword can’t even harm him anymore.
He had given up on life and tried to kill himself. But he had already gotten so strong he had achieved pseudo-immortality and even he couldn’t kill himself.
So for 500 years he was continually tormented by that sword. The sword took everything from him.
It took his friends, his family,and his lovers.
And so the demon king sits in an empty castle in seclusion waiting for the next hero to come to their death.
Until one day a young hero girl came to his castle….. without the holy sword.
She was apparently a newly summoned hero that wasn’t deemed worthy by the sword.
This was the happiest moment of the demon King’s life