Crusader Kings II: Sword of Islam

Summary pulled directly from Steam - Some text may only be applicable to Steam platform

Europe is in turmoil. The lands are fragmented into petty fiefs, the emperor struggles with the Pope, and the Holy Father declares that all those who go to liberate the Holy Land will be freed of their sins. Now is the time for greatness!
The first expansion, Crusader Kings II: Sword of Islam, will add playable Muslim rulers to the game.
Crusader Kings II explores one of the defining periods in world history in an experience crafted by the masters of Grand Strategy. Medieval Europe is brought to life in this epic game of knights, schemes, and thrones.
"The Ummah is under attack. All true Muslims must be charitable in peace and relentless in Jihad. Let the Sword of Islam fall upon the infidels that threaten our lands, our faith!"

Key Features:

Muslim rulers now playable
Updated interface for playable Muslims characters
Unlocks more than 20 features in the game
New counties and titles
Balance Greed vs. Nobility with the new Muslim Dynasty Decadence System
Polygamy. Allow up to four wives for Muslim rulers.
Story events, starting circa the year 100, such as: The Harem, Shia vs. Sunni, Sufi mystics, Dervishes, The Assassins, Sharia, Wife wants to become first wife
Combat Revisions with new Commander traits for more interesting combat tactics