Prison of Night: The Dumarest Saga Book 17

ZAKYM: World of the night-rulers and the day people. A world teetering on the edge of nightmare. The Cyclan orders were clear - 'Find Dumarest. Find him before war erupts on ZAKYM.' But Zakym could hold the key to Dumarest's search for his lost home planet - and for that he would risk everything. (First published 1977)