Crusader Kings II
Crusader Kings II is the sequel to Crusader Kings and is developed by Paradox.
Like its predecessor, it’s a grand strategy game, where you take control of a medieval dynasty from 1066 to 1453 (although with DLCs it’s possible to start as early as 769). You further your dynasty through the strategic use of war, marriages and assassinations amongst many other things.
Interestingly, there is also a save converter available which enables to you to convert you Crusader Kings II save file into a mod for Europa Universalis IV – meaning that you could start a game in 769 and play all the way through to 1821.