ShackNews has an interview with Mike Verrette about Titan Quest: Immortal Throne.
Shack: Without giving away the ending to the original game, what is the story line for the Immortal Throne expansion?
Mike Verrette: The story in Immortal Throne will pick up almost immediately where the original Titan Quest left off. Just when you thought you have saved the world from infinite peril, infinite peril arrives again. In Immortal Throne, players will be tasked with seeking entrance to the underworld and discovering the source of a demonic army that is threatening the land of the living. As the player journeys through Hades he will be advancing through wave after wave of the new horde in an attempt to find the source of the advancing army.