In the aftermath of the defeat of the King of Shadows, the Sword Coast is in the midst of an uneasy economic recovery. Trade syndicates have sprung up to exploit the post-war confusion for their own ends. The player’s party will wade into this uncertain environment; in order to increase their own fortunes, they can either ally with a syndicate to create a trade empire, or cut their own path through Faerûn by preying upon caravans and selling the goods on the black market. As they attempt to extend their influence, players will become aware of a new faction working behind the scenes: the evil, shapeshifting, serpentine Yuan-Ti.

defending the castle is very close to the end of the original campaign. Then you move on to mask of the betrayer.

This new addon is a whole new story though to introduce the D&D 4.0 Ed rule set.