Charon: A Dragon at the Gate — Jack L. Chalker

ASSIGNMENT: HELL WORLD

They took the body of Park Lacoch and stripped away his mind, transferring the mind of a top Confederacy operator to his brain. Then they stuck him aboard a spaceship and exiled him to Charon, one of the Worlds of the Warden Diamond, from which no man could ever return. A mysterious organism on those worlds infected everything, making life away from them impossible afterward.

Charon was a hell-world in literal fact. It was too near its sun, making life all but unbearable in its hot, steamy atmosphere. Strange beasts roamed its jungles--and stranger still, the group known as the Witchcraft plotted with the Lord in exile to take over the planet.

There Witchcraft was real--for on Charon, magic worked! The Warden organism could be made to perform almost any feat under the control of a skilled witch or sorcerer--as Park Lacoch discovered when he was transformed into a changeling--a half-beast half-man, with the beast rapidly gaining ascendancy.

After that, his assignment really became difficult...