The movie is based on the graphic novel Hercules: The Thracian Wars
Here is a synopsis of the graphic novel
Fourteen-hundred years ago, a tormented soul walked the Earth that was neither man nor god. Hercules, powerful son of the god king Zeus, received nothing but suffering his entire life. After twelve arduous labors and the loss of his family, this dark, world-weary soul turned his back on the gods, finding his only solace in bloody battle. Over the years he warmed to the company of six similar souls, their only bond being their love of fighting and the presence of death. These men and woman never question where, why, or whom they go to fight; only how much they will be paid. Knowing this, the King of Thrace has hired these mercenaries to train his men to become the greatest army of all time. Hercules begins to question King Cotys’ motives when he takes his army out to battle and sees them practice on innocent men, women, and children of their neighbors. Deep in his soul something stirs, but is it enough to stop a mad king and his army of the damned from marching across Greece – or even Olympus itself?!