The Black Phantom!
The first step must have started with that black fox. Waking it up was the key, and it must be done before doing anything else.
Bringing the black fox out of the cage would serve as a distraction for that spirit, letting it move as fast as possible to stop it from causing problems. That wouldn't just serve as a distraction, but would also let that spirit show itself, bringing it out of its hiding spot.
It wouldn't imagine the presence of such a predator here. In worst cases, the Black Phantom would scare that spirit away, but William wanted it dead.
Like this, he'd send a clear message over to the ones who caused all this to him. And that would also come with lots of benefits at the end. After all, this was a spirit, one which knew lots of hidden matters in the spirit world.
But first he had to see if the Black Phantom would be able to kill it or not.