Dark Prince — David Gemmell

The chaos spirit had chosen the child Alexander to be its human host. But Parmenion, most powerful warrior of ancient Greece, had won a small victory over the darkness that sought to rule through Alexander. The boy's soul had not been destroyed by evil, but instead had merged with it -- and now Parmenion aided Alexander in the battle between light and dark that constantly raged within him.
But there was another world, where the creatures of Greece's legends still flourished. There, the chaos spirit already ruled, through a demon king. In this Greece, there was a prophecy that a child of great power, the legendary golden child, would come and restore the fading magic of the land to the creatures of myth. The demon king believed also that devouring the heart of this fabled child would give him immortality. He believed Alexander, with the power of the chaos spirit within him, to be that child. And so he called Alexander into his world . . .
Only Parmenion, guided by the seeress Derae, his lost love from another life, could hope to save Alexander from the demon king. But who could save the young prince from the chaos spirit that threatened to conquer his soul?