I'll throw an idea into the mix! The enemy is a new force entering the fray, a mysterious order of knights who have sworn to end the depravity of the current war by striking down both sides with zealous might. Perhaps they've raised a host of the disenfranchised to do so: peasants whose farms were burnt, mercenaries, etc. I'm imagining that they'd be deeply flawed, corrupt, or overzealous. Some might be cynically playing their true believer comrades while they sack and rape across the countryside. Others might be genuinely faithful but believe that their righteousness justifies any means. Others might just be out for vengeance for a wrong done to them.