The X Factor — Andre Norton

Diskan Fentress, mutant son of an interstellar explorer, felt himself out of place on the luxury worlds of civilized space. It was not until he stole a spaceship and a travel key to the unexplored planet Mimir that he first felt his latent powers to be of some worth. For Mimir was a world of strange relics, of beasts that were more than beasts, and of a ruined city whose shadows bespoke a higher and weirder civilization than any the galaxy had yet discovered. It was to fall to Diskan's lot to meet head-on that ancient world's "X Factor," and to settle the fate of a planet with his reaction.