Diablo Expansion: The Rebirth has no a proper storyline, infact the idea is that the players will have the role of continuing it, restoring the glory of Diablo and the whole scene.
By the way, ReX takes place after Diablo II, willing to call back all the heroes that landed away from Tristram.
In the "new town" you will find an undead Griswold, brought back to "life" by Pepin, who died for this task. Also Decard Cain left the sceptre of the wise to Alain Cain, the last known Horadrim.
Me and everyone involved in this project hope that ReX will bring new passion for the great world of Diablo, and remember...the story is about to be written, let's taking part of it!