Ancient Earth, book 2: A Hero's ThroneRoss Lawhead2013

Deep beneath the streets of England lies another realm . . . one few in our modern world know exists. Daniel and Freya, however, know it all too well. Eight years ago, these friends first journeyed through portals into the hidden land of Niðergeard - discovering a city filled with stones, secrets, and sleeping knights that serve to protect the world they call home.

But Niðergeard has fallen to dark forces, overrun by its enemies. Gates are being opened between the worlds that should have been kept closed. The battle lines for the war at the end of time have been drawn, and opposing forces are starting to gather.

Having served for centuries as the first and last outpost at the borders to other worlds, Niðergeard must be reclaimed and the mystery of its fall discovered. Daniel and Freya, along with an ancient knight and a Scottish police officer, must return to the legendary city, rally the surviving citizens, and awaken the sleeping knights - knights who are being killed, one by one, as they sleep.

But time is running out faster than they know.