The Ikko Ikki are religiously inspired rebels.
Ikko Ikki may emerge at random at a district where a riot has occured. The odds of emergence of Ikko Ikki tend to be higher in districts with a famous temple.
Once Ikko Ikki are established in a district they will create armies, to defend their towns. Their towns are often heavily fortified. The Ikko Ikki tend to build high level castles and temples in districts where they have been established for some time.
Ikko Ikki who are left unscathed by war, may spread to neighbouring districts. They tend to spread like an oil stain. If the Ikko Ikki are left untouched, their territory will eventually get larger and larger. As their realm increases in size, their spread will be more rapid. So it is best to take them out early, if they are near you, or else they can become a real nuisance.