Jewel in the Skull — Michael Moorcock

"Those who dare swear by the Runestaff must then benefit or suffer from the consequences of the fixed pattern of destiny that they set in motion. Several such oaths have been sworn in the history of the Runestaff's existence.
--The High History of the Runestaff

Dorian Hawkmoon, late the Duke of Koln, fell under the power of the Runestaff, a mysterious artifact more ancient than Time itself. His destiny, shaped by a vengeful oath sworn by the maddened Baron Meliadus of the Dark Empire, pitted Hawkmoon in battle against his own allies and forced him, by the Black Jewel embedded in his skull, to betray his very heritage.