I thought the story was awesome and actually, they could have maybe even made a full-on game about this concept. However, things are not good as the brother of Diablo, Baal is causing havoc and you need to head to the lands of the Barbarians, destroy the World Stone and save the day. You may wonder how you carry on after what happened with Diablo at the end of the main game.
The story of Diablo II: Lord of Destruction is awesome and actually way deeper and more interesting than you may expect from an expansion.