In the end, a big gap was created between them and Sloth's army. In their eyes, such a gap wasn't a problem as it'd take a few hours for their masters to cross. But they missed something crucial here.
Sloth's army was going to dominate and rock the entire Black Hole area thanks to William's plan. So Sloth and others weren't worried or even irritated by this. In their eyes, the incoming armies would find nothing at all to do except for dead bodies of dark masters and liberated zones.
As things were getting to such a point between the colossal master armies, William finally consumed enough spirit power to activate the heart.
The moment the heart got activated, the insane pressure he was suffering vanished. It happened in mere seconds, and then he got the chance to finally check what was going on here and everywhere around.