Robert Silverberg 
Man In The Maze 

During his heroic first encounter with an alien race, Dick Muller was permanently altered, hideously transformed in a way that left him repulsive to the entire human race. Alone and embittered, he exiled himself to Lemnos, an abandoned planet famed for its labyrinthine horrors, both real and imagined. But now, Earth trembles on the brink of extinction, threatened by another alien species, and only Muller can rescue the planet. Men must enter the murderous maze of Lemnos, find Muller, and convince him to return with them.But will the homeless alien, alone in the universe, risk his life to save his race, the race that has utterly rejected him? (First published 1968)