Pontos in my campaign has gone on a rampage as well. All of Anatolia belonged to them minus Pergamon, which retained independency (even though occassionaly faced with an assaulting army). That is, until Basileus Alkyoneus "Philopator" decided to thwart their plans of further conquest and sent his son Perseus with a Royal Army.

I'd post a picture, but I'm quite bored. I don't think I remember seeing any other "underdog" faction become too powerful. Maybe the Sab'yn, in a campaign a guy posted over at the Faction Progression thread. Those desert monkeys managed to capture Antiocheia and hold on to it in his campaign.

Maion