Judgment on Janus — Andre Norton

NAILL RENFRO
Space-born and damned by the savage ironies of war to the most unendurable of all thralldoms, he knew that life in bondage on Janus would be the worst of a galaxy of hells. What he didn't know was that freedom on Janus could be the same. Then he unwittingly committed the ultimate sin....

AVYAR, THE SWORD BEARER
Brought to life through a monstrous metamorphosis, he could remember only faintly his life as Naill Renfro, and could barely comprehend the race he'd become part of. All he knew was what the Iftin allowed him to sense -- and what he sensed was more terrifying than anything he'd ever known....