Michael Moorcock 
Jewel In The Skull 

Dorian Hawkmoon, the last Duke of Koln, swore to destroy the Dark Empire of Granbretan. But after his defeat and capture at the hands of the vast forces of the Empire, Hawkmoon becomes a puppet, co-opted by his arch nemesis, the ruthless Baron Meliadus, to infiltrate the last stronghold of rebellion against Granbretan: the small but powerful city of the Kamarg. He has been implanted with a black jewel, through which the Dark Empire can control his every decision. But in the stronghold of the Kamarg, Hawkmoon discovers the power inside him to overcome any control, and his vengeance against the Dark Empire is filled with an unrelenting fury.