The Forgotten Legion by Kane, Ben

Romulus and Fabiola are twins, born into slavery, and then later sold: Romulus to gladiator school, and Fabiola into prostitution, where she will catch the eye of one of the most powerful men in Rome.
Tarquinius is an Etruscan, a warrior, born enemy of Rome and trained by the last haruspex in the forgotten arts of divination. A runaway slave, then an AWOL Legionaire, he has a long foretold destiny that will take him to the ends of the known world.
Brennus is a Gaul who lost his entire clain in a battle against the Roman Army. After being sold as a slave he rises to bcome one of the most famous and feared gladiators of his day.
The lives of these four are interwoven in story that ranges from a Rome riven by corruption, violence and political enmities to ends of the empire, where Romulus, Brennus and Tarquinius find themselves fighting against the Parthians and overwhelming odds – members of The Forgotten Legion