User blog comment:Ralnon/WHEN is the Opening Cinematic?/@comment-5136207-20120603234135/@comment-58609-20120604001533

Great Conflict, not Sin War.

Technically, it could be set at some point in the future due to Cain's monologue about the EoDP, and that Leah is experiencing it as a vision (prophecy?), but I doubt it somehow, if only for the fact that Diablo and Hell were distinctly defeated in DIII, and I don't see them returning at least for the expansions.