Duty affects Commanders only. Has higher affect on Warlords, highest on Chief Warlord. Requires them to use their own initiative in the service of the Ruler. They can't withhold information (though Parson did manage to work incognito when he was convinced it'd be more effective) and can't conspire against the Ruler.
Rulers may be affected by some form of Duty that makes them pop or designate heirs, an action that is good for their side but has no advantages for them (and can even be a dangerous option in some cases).