In the second to last chapter of Isaiah, we get a vision of the Holy Mountain, of that promised reign of peace that's going to ultimate and permanently be so awesome. But in the very last chapter, after some initial statements about God vindicating and comforting Zion, we get a darker vision. Isaiah ends on a cautionary note, predicting the how the wicked will suffer the full brunt of God's wrath.
It's a somewhat macabre image—the good, peaceful people will be able to go out and look at the dead bodies of the not-so-peaceful people, rotting in the fields after being destroyed by God:
And they shall go out and look at the dead bodies of the people who have rebelled against me; for their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh. (Isaiah 66:24)
It seems, after wrath leading back into mercy throughout the course of Isaiah's prophecies, the book would end with a final, optimistic statement—and it almost does. But it feels it necessary to qualify itself, to say: "But if you don't live up to what's demanded of you, you lose out in a big way."