That mission is rather unpleasant.
It looks like the event sorts your provinces by population and makes a rebellion with the most populous 1/3 of your empire where the governor is not of your dynasty (or of your dynasty with a loyalty of less than 60). It also causes all subjects with loyalty less than 60 to be subjects of the rebellion.
I had to do a bit of a governor shuffle and wait to integrate Bactria (whose ruler apparently had a trait making it relatively disloyal) to make the civil war doable. I'll probably never click on that mission task again.
It looks like the event sorts your provinces by population and makes a rebellion with the most populous 1/3 of your empire where the governor is not of your dynasty (or of your dynasty with a loyalty of less than 60). It also causes all subjects with loyalty less than 60 to be subjects of the rebellion.
I had to do a bit of a governor shuffle and wait to integrate Bactria (whose ruler apparently had a trait making it relatively disloyal) to make the civil war doable. I'll probably never click on that mission task again.