This is story related. After fighting your way through the arena telling the "true" story of Commodus' victories on the battlefield, you will finally get to face the self acclaimed god. This fight is broken into three segments. First it will play out as a standard fight. Dodge his unblock-able moves and attack or heavy shield bash till his health is roughly at 20% and a cut scene plays. There will now be several impostors that you will have to kill before continuing your fight with Commodus and breaking down his health away. He seems to be slightly more aggressive on this fight so be more defensive and evasive rather than forcing your attacks on him. Another cutscene will play, resulting in Marius being poisoned. You must counter two attackers (their deaths will be automatic) before fighting Commodus for the last time. Follow your previous steps and let him breathe his last breath.