Total War: Warhammer III

You wont have to go out of your way for this. For one you get skulls for every battle and as Khorne, you want to battle, a lot. If you want to speed it up, raze every settlement for skulls and regularly summon enemies for you to fight with Skarbrands army ability on the campaign map. I unlocked it around the time before my fourth soul without going for it in particular.

Thanks for OneWingAngel for the reminder: This is not cumulative. You need to have it at once.