It was a perfect chance for him to go and kill that monster once and for all. And yet the scrolls never mentioned where he went, as if the dark masters got relieved for getting rid of that nuisance and the scrolls returned to talk about war preparations and other stuff.
William didn't lose hope to find any mention of his nemesis' location, but he was destined to get disappointed in the end. The scrolls ended by the middle of the war events, without any additional information about that monster.
"It's blessed with damn luck that I don't know where it got… Damn!" William was quite frustrated by this. He didn't know why that monster turned on its people, but it was the golden chance for taking it down.
What he didn't know was that he was the root of all this. The Nightmare monster was well aware of what happened to William, and knew about him going to another realm.