The Stallion that Mounts the World

Every county on the map, starting as Temüjin. The real Genghis only managed a chunk of it; you're expected to finish the job. Phase one is This Chinggis Everything - the full Greatest of Khans playbook (Nerges on cooldown, Dominance, the Great War of Defiance). Then the world tour: swap your regiments to Keshig and Cataphract Archers (available via Khitan or Tangut vassals), lean on the Offer Submission interaction to absorb realms without fighting every single war, and hand conquered land to local-culture rulers with high Martial so the empire holds together behind you. Wiki-rated very hard and among the rarest in the game - this is the campaign you tell people about afterwards. Expect it to take a very, very long time.