Paradox Interactive has released Crusader Kings II: Sword of Islam, the first expansion for the Windows and OS X editions of Crusader Kings II, their strategy/RPG sequel. The expansion is available through digital distribution for $9.99/€9.99, and this page lists where it can be found. The add-on is accompanied by a new version of the game, offering "improved combat mechanics, numerous new events, traits and decisions." You can find the full patch notes on Steam. Here's word on the expansion:

Thought keeping an eye on one wife was bad? With the release of the Crusader Kings II: Sword of Islam expansion, this scheming strategy RPG will allow players to command Muslim nations, opening up a world of polygamous problems, new laws, traits, titles and over 100 new events, as well as a flavorful Islam-inspired interface. Experience the Crusades from the other side. Combat decadence within your own dynasty. And solve problems arising in your very own harem. Or just start panicking and invade France. Your call.

Crusader Kings II: Sword of Islam enables players to play as any Muslim ruler across the great dynasties including the Ottoman and Mamluk empires. These brand new gameplay mechanics reflect the distinctive traits of the great rulers and empires of the time, creating an entirely new experience for players and introducing another layer of richness to one of the finest strategy games of recent years.