In EU4 you control a country. In CK3 you control a dynasty, one ruler at a time.

EU4 plays like a complex but fairly “normal” strategy game in a pseudo historical setting. CK3 is more focused on managing your complex web of feudal relationships. Almost dating-sim.

My best game of CK, I started as a minor a Norwegian noble and slowly worked my way up to becoming king of Norway, also owning part of Denmark. But I pissed off too many other nobles along the way, and they schemed with my brother to depose me in favor of him. I then went on a 20 year revenge spree to assassinate all of his heirs and inherit the kingdom back from him!

I recall the trailer for one of the CK2 expansions had a bullet point feature that could really only happen in CK: “SEDUCE YOUR RELATIVES!”