The Hero of Ages by Brandon Sanderson
This final volume of the Mistborn Trilogy (see reviews of book one and book two) brings its world to the brink of Ruin. Ruin being the god-like conscious elemental force that Vin released from the Well of Ascension at the conclusion of the second book. Ruin, as its name implies, seeks the destruction of the world. In ancient times, Ruin and its opposite, the god-like conscious elemental force called Preservation, made a deal where they worked together to create the world. Preservation wanted a special creation, man, so it gave extra of itself to create human beings with the freedom to conserve or destroy. Ruin wanted something extra as well--to destroy the world they had created. They agreed in ancient times but later on Preservation trapped Ruin at the Well of Ascension so that mankind could live perpetually. Preservation gave up almost all of its consciousness to accomplish this imprisonment. Now that Ruin is free, the end of the world seems at hand. The ashmounts are throwing more ash in the air, blocking out the sun and covering up plants so the people can't grow crops. Earthquakes are unnaturally frequent. The mists come out earlier in the evenings and stay later in the mornings, also causing problems for crops and for people afraid of the mist-sickness that has been killing a few and leaving many sick for weeks.
Vin and her husband, Emperor Elend Venture, fight to stop all the havoc. They've discovered secret messages from the Lord Ruler in special caverns he stocked with supplies for just such an occasion as the end of the world. It turns out the Lord Ruler (who certainly seemed like a villain in the first two books) was preparing for his eventual fall and the release of Ruin. He wanted to give the world a chance to fight back. He created the koloss as powerful fighters and the inquisitors as religious leaders. Ruin has been secretly undermining that plan and uses the koloss and the inquisitors to bring about the end of the world. Ruin has been literally rewriting history--anything written on paper can be changed by Ruin to read differently (that's how Vin was tricked into releasing Ruin at the Well). Only words written on metal (such as those left in the special caverns) can't be changed. How can our heroes win out against such a powerful and nefarious foe?
The book justifies the five hundred or so pages of world building I complained about in the second book. The different creatures (koloss, inquisitors, kandra) and the larger world are all brought into context and significance in this final volume. Seemingly insignificant details help to explain the larger story and bring about a satisfying if not very upbeat ending. I enjoyed this book and the trilogy thoroughly.