An officer unit recruited by the more savage and less socially organised of all factions in Kings & Conquerors, the Chieftain is the replacement for the Centurio and the Taxiarchos. He doesn't have the armour of the Centurio, or the marching speed of the Taxiarchos, and certainly cannot use the Entrench ability, but he doesn't have to since he can boost the attack of nearby units by +2, making lower-level units within his area of effect somewhat more formidable. Equally, the chieftain is also cheap, meaning that a vast barbarian horde led by these commanders is always a possibility.
Chieftains have a cost structure that is wholly different. Whereas the taxiarchs all need metal, food and gold, Chieftains are recruited solely on gold and influence - a base cost of gold, followed by a ramping cost of food and influence.