What Nepotism?
Very rare1.9%ObjectiveOptionalSingle-player
Have your Dynasty rule ten independent realms of at least Kingdom tier simultaneously
Ten independent kingdom-tier realms all ruled by your dynasty at once. The wiki's clever start: in 867, replace the Kirghiz khagan with a custom ruler with nine sons. The Kirghiz fully control the de jure land of three kingdoms and half of two more - and crucially the Khanate itself has no de jure territory, so you can hand out kingdoms' independence without dismantling your own title. Create the kingdoms you control, conquer a few more, grant one to each son, then release them all as independent in one glorious act of nepotism: you plus nine kings = ten. Keep everyone alive until it counts.