[johnny]

You know, I think the pantheon civil war might be the easiest plot-wise. We could have people on earth become vessels of sorts for the Egyptian gods, which created a war and ended civilization as we knew it.

And maybe some gods who weren't exactly invited to the party (we could combine options one and three here and say the battle between Set and Ra is what happened on earth, and the other gods are angry about it) want to clean up the mess (or take it over) so they get their own vessels to walk about on earth.