Neverwinter Nights 2: Mask of the Betrayer takes you back to the events of Neverwinter Nights 2 just after the defeat of the King of Shadows. This new story concludes the events from that game, but it isn't necessary to have beaten it, or played it, in order to run through this new expansion pack. Though, like any well designed expansion, even if you don't have to play through the original in order to start in this game, there are benefits.
For players who have gone through Neverwinter Nights 2, you can import your characters and if they haven't already gotten up to level 18, then they are automatically upgraded. If, on the other hand, you've done the extra work and gotten your character higher than that, then you will have a bit of a head start in the new campaign. If you don't have a character or you want to start a new one from scratch, then this new character will start off at level 18.
One of the benefits to starting a new character is the ability to play as a new subrace, the Genasi. These are elemental/human hybrids that come in four flavors: Earth, Water, Fire and Air. Each of these characters has their own skills and looks that are very fitting to their heritage.
The story of Mask of the Betrayer takes place in the, yet unexplored, land of Rashemen near the city of Thay. It also gives you access to two new base classes: the Spirit Shaman and the Favored Soul.
Look for this expansion pack to hit shelves this Fall.