Jandar, the alien lost in the mysterious and treacherous domain of the insidious Mind Wizards. 

Searching for him is a fearless armada of his loyal friends, and a stranger who knows him better than anyone on Callisto. The stranger, Lankar, is none other than Lin Carter, accidentally transported through the Gate Between the Worlds to the land of the Thanatorians. Aware of Jandar's plight, Lankar joins the search beyond the world's edge and suddenly finds himself savagely battling for his own existence against the most nightmarish creatures on the planet — the gruesome flesh robots of Kuur and the fiendish Mind Wizards of Callisto themselves...

This is Book #6 of the Callisto series. Jandar, formerly Jonathon Dark of Earth, is mysteriously transported to the savage Jovian moon Callisto.