A Game of Thrones is a total conversion mod for Crusader Kings II, based on George R. R. Martin’s fantasy novel series, A Song of Ice and Fire.
This review may contain spoilers
Expertly executed in general, and as the description said, allows you to explore some very interesting "what if" scenarios. In some games, House Baratheon may not even exist, if you manipulate events early enough. When the Starks are the LPs of the Reach and the Lannisters have both the Stormlands & The North (from After the Spring start), the the mod becomes completely unpredictable.
Beware, for you could lose all you've worked for when they stir beyond the Wall.