with all this talk about nuclear activity that went on earlier, i feel like the last three songs will have sort of a chronology to them.
fallout could be the start of an apocolyptic feel, referring to fallout shelters.
then the catalyst can be the outburst of the explosion, as i think it has shaped up to be a very big song. big as in lots of screaming, heavy drums, and overall really intense.
and than the messenger can be the outcome of it all. maybe a 'last person on earth kind of deal.'
/just a quick speculation, i could be way off, but i thought i'd share haha just for fun.