Dominions 3: The Awakening/Units/Spy

As with scouts, spies are stealthy commanders who typically have minimal combat or leadership ability. They do, however, normally have even better skill at penetrating enemy territory undetected. They provide more detailed reports on enemy assets within the area. In addition, they may be ordered to agitate within a province, inciting unrest against the false pretenders.

New units can't be recruited in provinces where unrest is greater than 100. So sending multiple spies to an enemy's capital to incite unrest can shut down enemy army production. This can be especially effective if he depends on sacred or national troops for the core of his army.