Dave Duncan 
Demon Rider 

In medieval Scotland, an outlaw seeks his freedom—from both the merciless khan who conquered Europe, and the demon spirit that possesses him. All of Europe is ruled by the Khan, whose Golden Horde swept its conquering way across Europe in 1244. The Scottish outlaw Toby Strangerson, known as Longdirk, is ruled by an even harsher master. He is possessed by a “hob, ” a demon spirit murderous as a child, although inherently neither good nor evil. Toby wants his freedom—and the spirit of the tyrant demon Nevil, ensorcelled in amethyst, can be traded for the exorcism of the hob. In order to make the exchange, though, Toby and his friend and ally Hamish must face the hexer Oreste on his own ground, in the dank and fetid dungeons of Barcelona, where souls are racked and tortured and destroyed on the relentless wheel of the Inquisition.  This book was originally published under the pseudonym Ken Hood.