This puzzled William a lot, knew he had to go to the man himself and force him to spill out his secrets to get answers. But William didn't even think about this. It wasn't because he was weak, but because spirit masters used to have their secrets, a rule that William respected.
And yet that wasn't the crux of William's problem, the technique he acquired from that black fox was!
William didn't expect such technique to run over his accumulated spirit power. It required him to sacrifice part of his spirit power to elevate his mastery of this technique. And like this, even with the presence of that mysterious master's pocket worlds and tons of monsters, even with William's Devour ability, the latter couldn't help but feel frustrated every single time he sacrificed part of his hard gained spirit power.
The only reason that stopped him from throwing away this technique or halt his training in it was the grand benefits he gained.