Might be a nice idea to allow the summoning of Magic beings to subvert a city slowly? Could base it as a city enchantment and each has a negitive effect on the city and positive effect for the caster.
Nature. A elder dryad and her tree begin subverting some of the populace. Sawmills/blacksmiths are summarily sabatoged and forests are planted by the cultists.
Death. Dark Ritualists. A small cult of your Ritualists begin a campaign of murder, dragging sacrifices to their alter. Population growth is hurt and a small trickle of mana is given back to you based on how many killed (Gives very little for a small city, or more then it costs to maintain for a large city)
Chaos. That old rebel spirit. These cultists simply like breaking things. Oh, and change. City walls become useless, and attempts to change the ruling power of the city go much better.
Sorcery. Cult of Secrets. These folks love their magic, draining local production and keeping their ears open for you. Feeding research back.
Life. Life dosent have cultists, they have missionarys
Law. Perhaps somthing to cast on your own city, The Justicar run the city with an iron first, increesing general unhappyness some, but wiping out other cults in the city.
Option: Cast and forget. But dont expect them to leave if you capture the city later.