The Shadow of the Torturer by Gene Wolfe (1980)
A far distant future where the sun has cooled and people live a medieval existence in the ruins of a once technologically advanced world. The main character is a member of the torturers guild who commits the crime of showing mercy and has to face the consequences. It's a bleak—really bleak—mix of sci-fi, fantasy and horror with an overarching mood of melancholy and decline. I'm about halfway through and it's pretty interesting so far.