E.C. Tubb 
Spectrum of a Forgotten Sun 
The Dumarest Saga Book 15

Earl Dumarest, now a vanquished and captive mercenary, is offered the choice of surrender to the fearsome Cyclan or the chance of freedom by the beautiful and ruthless Lady Delphine. The risks are terrifying — but has Dumarest a choice?Intent on finding the legendary world of Terra — Earth — the planet of his birth; pursued by the brilliant and ruthless megalomaniac race, the Cyclan, whose secret key to Galactic domination only he possesses; trapped on a plague ship hurtling through a universe determined to destroy it, and captained by a madman — Earl Dumarest is tempted by the irresistible but fatal promise of Delphine.The choice is simple — to abandon his dreams or perish by them, just as they promise fulfilment.(First published 1976)