Dark universe - Daniel F. Galouye

The survivors live deep underground, as far from the Original World as possible. It?s true that some of the hot springs that sustain life are failing, and they are plagued by the huge and vicious soubats, and subject to sudden raids from other tribes. But they are safe from the Twin Devils, Strontium and Cobalt, and protected from the ultimate evil, Radiation. Then something strange and frightening starts to happen: terrible monsters, who bring with them a screaming silence, are seen and people are disappearing. One young man knows that to find out what?s going on he must question the orthodoxies of his faith and defy the law. He must discover the nature of Darkness itself...