Gregor and the Bloodbath of Regalia

It's been four years since Gregor has ever uttered the words "fly you high," rode a bat, taken part in a prophecy, or been to the kingdom of Regalia in the Underland.
His family saw it as a blessing to finally be done with that world, that chapter of his life.
Well, everybody but him.
Though the scars on his legs and arms were fading, there were scars he knew even time couldn't heal. Not with how his life was now, on the farm in Virginia.
But one day he runs into an old friend: A now twelve-year-old Halflander child; Hazard, who tells him that Regalia is once again about to be thrown into war--
And calls for Gregor's help once more.