Each Ermor-controlled province should have a temple. If the provinces are not connected, the dominion will spread further and faster, and Ermor has multiple force-gathering points.
One thing I'm not sure about is if the Ermor AI will do suicidal attacks, and leave a fort without mage (and thus without further "recruitment".
Dom:PPP had a scenario in which Ermor was the Big Bad Guy. Ermor had multiple starting point in a big map, each province had special leaders, magic site(s), fort, etc. I can post Steerpike the details if you want to. The map would better be big, though, as most of the Death artifacts that mention their wielder's name come from that spesific scenario...