The Ladder in the Sky — John Brunner

A young man must deal with the consequences of a demonic possession -- from the science fiction author whose “best writing is turbo-charged with ideas” (BBC).

In The Ladder in the Sky, a starving youth, trapped in poverty and with no hope of escape, is taken prisoner and offered up in an actual “deal with the devil,” -- servitude for a year and a day in return for helping a resistance group free their imprisoned planetary leader. When he returns to consciousness, he is told that the devil has taken up residence inside him. At first, he thinks nothing is changed and he can take advantage of the situation but some upsetting surprises are in store for him. With an SF setting and a fantasy premise, this is one of Brunner's best hybrids of action, magic, technology and suspense.
 
“One of the most important science fiction authors. Brunner held a mirror up to reflect our foibles because he wanted to save us from ourselves.” -- SF Site