Terrifying doesn't begin to describe it. Despite the author's best efforts, no words can. We as a species quite simply lack the capacity to comprehend the horror by several orders of magnitude, else the world would not look like it does. Just how long can a doomsday clock hold at 90 seconds to midnight? The world can end at any moment and it won't take much longer than an hour from the first warning just a split second after the first nuclear missile is launched to the end of everything.
Told in clear and authoritative detail, this is the story to end all stories. Because after this, there is nothing.