Crusader Kings II: Horse Lords

Main features:

• New Central Asian cultures: Tarim Basin and large parts of Mongolia added to map.
  
• Nomadic government available for steppe cultures. These function in a radically different way from sedentary societies - you manage populations instead of holdings, and armies are raised based on population size. Also, the ruler’s power must be balanced against the interests of the various Clans within the realm (Clans are not normal vassals, but rather distinct families and factions within the nomad group).
  
• Over a hundred new events add extra color and flair to the Steppes and Central Asia.
  
• Non-nomadic rulers will have special options for combating nomads, such as constructing forts in wild enemy lands.
  
• The Silk Road, with corresponding dynamic trade mechanics and conflicts.
  
• New nomad mercenaries. Send your unruly sons or brothers away as soldiers of fortune.
  
• Landless raiding adventurers may appear and harass a region for a time or try to conquer land and settle.
  
• New diplomacy option: Tributary States.