Hmm. I feel like he's going to be a 'dark' hero.
Remember the last hero had the ability to drive away the darkness with his light. Curious as to who that is - but regardless, John is known for his great display of darkness.
He shattered the crystal orb when it was brought to the village, and when he was taught how to use darkness it practically consumed him.
In this case would the sword instead consume the light? It'd be a blackhole instead.
That is unless I'm entirely overthinking this and the magic tool only has the one function that can't be affected.