When others saw this, they couldn't help but recall the deadly techniques of the high stage dark gold masters. Every single one of them was considered an army on his own, able to go within such swarms of monsters and kill their way through without any hindrance or danger.
As for William, he was delighted by such a result. He knew such technique was augmented by the newly acquired merge ability of his spirit elements. Thanks to using two fiercely hostile elements together, such a scary move was feasible and possible.
This technique wasn't as scary as his Holy Sword in terms of attacking a single or a small group of targets. But if he was in the middle of a huge army, the same situation he faced multiple times already, then this technique would be his killer move.