User blog comment:PrestonH/Cas in the Empty/@comment-31647781-20170902132004

Well, S13E04 is called the Big Empty, which is a bit of a giveaway if that's the answer. I don't think angels go to the Empty when they die because Billie did say nothing comes back from the Empty, and that means that Cas has already come back 3 times prior to her statement, which either is a continuity error or just doesn't make sense,