The poster also probably tells us that the Wall will be making its way down. The Wall, as you know, is the magical barrier between Westeros and the White Walkers' undead army. Not to mention the White Walkers themselves, but we'll save that. Should the Wall fall, which it might with some seriously technical magic, that would mean that the White Walkers will have free reign to invade Westeros. That would mean a gigantic bloodbath of historic proportions.