The Call of Earth

As Harmony's Oversoul (a sentient computer) grows weaker, a great warrior has arisen to challenge its bans. His name is Moozh, and he has won control of an army using forbidden technology. Now he is aiming his soldiers at the city of Basilica, that strong fortress above the Plain. Wetchik and his sons are not strong enough to stop this army, but meanwhile, inspired by a dream from the Oversoul, Wetchik's family prepares to return to Earth. (Google Books Summary)