Act IV Story

Your epic battle with Mephisto left him broken, but you were too late to stop him and his brothers Diablo and Baal from opening a portal to Hell, through which the two other siblings escaped. Diablo has once again reverted to his true form, and now prepares the armies of Hell for an unstoppable assault on your world. You must travel through the portal with your ally Deckard Cain so that you may destroy Mephisto's Soulstone and defeat Diablo himself.

You also find that Mephisto has been playing you and the forces of Heavan for fools all along, and that his imprisonment in the Soulstone was part of this plan. Mortals may plot and scheme, but the likes of the Three plot on levels that incomprehensible to us.