Lost Swords Ser.: Shieldbreaker's Story by Fred Saberhagen (1994, 1st edtion, hardcover).

Long ago, the gods forged Twelve Swords of Power and threw them on the gameboard of life for greedy mortals to scramble over. Too late, the gods learned that they had forged too well; the Swords could kill the gods themselves.Now, ages later, the Swords are back in human hands, as powerful as ever. Vilkata the Dark King returns from exile seeking revenge on those who defeated him. His first goal is the vault in which is enemy Prince Mark has placed the remaining Swords; with these in hand, none can oppose his rule. Only one untested warrior stands in his way: fourteen-year-old Prince Stephen, with Shieldbreaker in hand.