Warlords are warrior rulers of various lands and kingdoms. Thork was the Warlord of the Frost Orcs. Ancient warlords once foresaw that Graham would save his family from Mordack. Perhaps some may rule the Barbarians.
The 'ancient warlords' maybe those who ruled the disordered and unconsolidated lands of the kingdom of Daventry during the time of Edward's father, the old kingdom of Daventry. Many who lead the maurauding armies made up of enemies from within and without.