The first layer wasn't in darkness anymore. Actually, the first layer wasn't even a layer anymore. The earthquake was the aftereffect of the subspace dimension merging into the frontline.
After leaving May and Cole with their returned companions, he joined Jason and decided to return for good.
"We were actually inside when it happened," Jason said, "so we didn't notice much other than the darkness clearing out."
Jason had joined them along with the hundreds of soldiers who came to help with the forsaken problem, which was mostly sorted out by now. Apparently, the higher-ups worked fast on providing reinforcement, but the high time-dilation made even the fastest action outside ten times slower inside the subspace.
Ultimately, the reinforcement arrived when most of the problems were resolved, though not without taking a huge loss.
