E.C. Tubb 
Mayenne 
The Dumarest Saga Book 9

Earl Dumarest still seeks the mythical planet Earth . . . still roams alien and violent worlds. With him goes Mayenne, whose songs create joy and passion — or forgetfulness.Together they are cast up on Tormyle, a planet from another galaxy; a planet unique throughout the Universe. For Tormyle is sentient — the most powerful intelligence in the Cosmos, constantly recreating itself.Tormyle can be Paradise or Hell. Tormyle can manifest as a dragon or a knight on horseback, faceless behind the helmet. Tormyle understands nothing of humanity, of men and women, of emotion. And Tormyle will let no-one escape who cannot answer the unanswerable.(First published 1973)