Companions. The Parthian Chronicles, #5

They called themselves Companions - those who escaped from Italy with Pacorus in the aftermath of Spartacus' uprising, travelling across the Mediterranean to make their home in Parthia. They subsequently assisted Pacorus in turning the Kingdom of Dura into one of the strongest realms in the Parthian Empire. But one Companion was unwittingly left behind and condemned to the living hell of Roman slavery.
When Pacorus discovers this he and a band of comrades embark on a perilous quest to find and free him. Thus begins a journey that will see the King of Dura follow in the footsteps of Spartacus himself as he heads for gladiatorial games being held in the Roman-controlled city of Ephesus. This, the fifth volume in the 'Parthian Chronicles' series, is set in the time between 'Parthian Dawn' and 'Parthian Vengeance'.