Closer (Tunnels Book 4) by Williams, Brian

At the very centre of the Earth, in a world that no-one knows exists, Will believes he's safe.

But his enemy, the Styx, are close behind. They'll follow him to the ends of the world-any world. And after taking revenge, there's Dominion: their original plan to destroy all humans . . .

Back on top-soil, unexpected allies prepare to fight back. But as they unearth the secrets of the Styx, are they closer to the truth or death?