If you're bored of doing mundane things like "conquering settlements" in Total War: Warhammer 3, it sounds like the legendary lord being added to the Tzeentch roster in the upcoming Shadows of Change add-on might be for you. The Changeling is a shapeshifting daemon who, based on the latest Creative Assembly blog post, plays a totally different Total War.
We've seen other armies toy with infiltration mechanics before, particularly the skaven and Slaanesh, but it sounds like The Changeling leans in even further. After defeating a settlement, he's got the option to insert a trickster cult, seemingly returning it to the owner as you would after sacking a town. Only not, because from then on the cult can either work to funnel money to The Changeling while spreading propaganda to influence diplomacy, or encourage corruption throughout the region until reality has weakened enough for daemonic armies to come through.