The Lost Portal (Empty World Series) (Volume 4)

Lost technology might hold the key to a miraculous recovery. But some ancient devices have a mind, and an agenda, all their own.

Cory delves into the Empty World all on his own, with just a computer generated guardian for an ally. He knows that if he's ever going to walk again, he needs to unlock the forgotten secrets of medical technology advanced enough to heal him.

Meanwhile, the search party looking for Cory has troubles of its own. Lost in an underground city, Christy and the others have to keep one step ahead of the Ancients who are hunting them as they try to track down Cory. But is the guardian who controls those Ancients the same one helping Cory?