Crusader Kings II is a grand strategy game set in the Middle Ages, developed by Paradox Development Studio and published by Paradox Interactive as a sequel to Crusader Kings. It was released for Microsoft Windows on February 14, 2012.

The game is a dynasty simulator where the player controls a Medieval dynasty from 1066 to 1453, though the DLC The Old Gods and Charlemagne allow for an earlier start date of 867 and 769, respectively. Through the strategic use of war, marriages and assassinations among many other things, the player works to achieve success for his or her dynasty. The game contains numerous historical figures such as William the Conqueror, Charlemagne, Genghis Khan, Harold Godwinson, Robert Guiscard, Harald Hardrada, El Cid, Constantine X Doukas, Harun al-Rashid, Alexios I Komnenos, Alfred the Great, Baldwin I of Jerusalem, and Saladin, but allows for the player to choose less significant figures such as minor dukes and counts, and creation of entirely new characters with the use of the "Ruler Designer" DLC.

The simulation is open-ended, so the definition of success is completely defined by the player. The only in-game objective is to obtain as many prestige and piety points as possible in order to surpass the various historically relevant European dynasties in a fictional prestige ranking (the three most prestigious ones being the Capetian, the Rurikovich and the Habsburg dynasties). The game ends when the player's current character dies without an heir of the same dynasty to succeed him/her, when all landed titles are stripped from all members of the player's dynasty, or simply when the in-game year changes to 1453.

I can give you advices on how to play and tell you which family/kingdom to start with, especially if you are a beginner. I can also answer any questions related to the game. The game is available on Steam.

ps: I have not tried the mods yet, but there are many interesting ones like for instance a Game of Thrones mod.