A Chief is a unit that, belonging to a Natural Allies tribe, commands that tribe like the Ruler or a Warlord of a Side. All units from the Chief's tribe know his will by Natural Thinkamancy, even from other hexes. A tribe can have more than one Chief.
Unlike Rulers, a unit neither needs to pop as such nor being designed as an heir to become Chief. Being the last surviving unit of the tribe makes that unit a Chief. It is unknown how the succession works when all the current Chiefs croak. Perhaps the knight unit with the highest level becomes chief.