William couldn't tell how deadly such an attack was. He first needed someone to fight against to test this attack. Just judging from his feelings when he executed this strike and the narrow hole that was left behind wasn't enough.
"At least I managed to execute it…" he took a deep breath, stood up after one hour of rest and recuperation. "But it's not enough! I consumed most of my spirit power while stacking five slashes only. I have to lower that number and cut it by half first…"
He recalled his last night experience when he tried to lower the number of stored slashes while using his light based move.
He failed to trigger the attack when he used less than five slashes. "And what about this merge with the darkness around? Is it compulsory for the technique to get executed? But it got triggered on its own…"
He was careful to not pull such a move back then. However, his body moved on its own and merged with darkness.