The Hero of Ages

Vin and Elend spend the book racing against a collapsing world, following the Lord Ruler's hidden storage caches while Ruin—the ancient destructive force Vin unwittingly freed at the Well of Ascension—works to bring about total annihilation through ashfall, earthquakes, deadly mists, and control of the koloss armies. Sazed, devastated by the death of Tindwyl, loses his faith and searches through every religion he has studied for one that makes sense, while Spook develops strange new abilities from over-flaring tin and helps liberate the oppressed city of Urteau from the increasingly tyrannical Citizen, at great physical cost to himself once he discovers Ruin has been influencing him through Hemalurgy. TenSoon, imprisoned by the kandra elders, eventually convinces his people that the ancient prophecies of the world's ending are real, and the kandra resolve to help humanity rather than stand aside.

After Vin is briefly captured and then escapes the obligator Yomen at Fadrex City, Ruin reveals he can seize total control of the koloss and turns them against both Elend's and Yomen's forces. Vin draws Ruin's armies away to Luthadel, where she battles Marsh and the remaining Inquisitors, who are under Ruin's control. As she is on the verge of death, Marsh briefly reasserts his own will and rips out the Hemalurgic spike disguised as Vin's earring, freeing her to draw on the true power of the mists—Preservation's power, since Preservation has just died. Vin ascends and becomes the new vessel of Preservation, becoming trapped on another plane of existence locked in opposition with Ruin.

Elend leads the remaining armies of humanity to the kandra homeland at the Pits of Hathsin, realizing too late that Ruin has been manipulating him and Vin all along into bringing him straight to his own "body"—the vast atium stash hidden there. Surrounded by koloss, Elend discovers the mists have secretly been Snapping people into Mistings, giving him an army of atium-using allies for a desperate last stand. Marsh confronts and fatally wounds Elend in single combat, but as he dies Elend reveals that his soldiers have already burned away all the atium in the battle, permanently denying Ruin the power he needs to recover. Vin, now embodying both Preservation's power and, through her human nature, an echo of Ruin's destructive capacity, attacks Ruin directly at his weakened core, sacrificing both herself and the last of Preservation to finally destroy him.

In the story's final twist, it is revealed that Vin was never actually the prophesied Hero of Ages—Sazed is. The prophecy that the Hero would "bear the future of the world on his arms" referred to the vast stores of knowledge held in Sazed's feruchemical copperminds. Regaining his faith, Sazed claims the combined power of the now-dead Preservation and Ruin from Vin and Ruin's fallen remains, and uses it along with his accumulated knowledge to remake the ruined world into a lush paradise of blue skies, green plants, and flowers. The surviving characters—Spook, Ham, Breeze, and others—emerge into this new world and find Vin and Elend's peaceful, lifeless bodies lying together in a field of flowers. Sazed leaves behind a written record explaining his transformation into a god-like Hero of Ages and the true history of Preservation and Ruin, along with a private note to Spook revealing that Sazed made him a full Mistborn, healed the damage his tin-flaring had caused, and hinted that two more metals and their alloys remain undiscovered in the world. The book closes with Spook stepping into a leadership role to guide the survivors in rebuilding civilization in this transformed world.

By the end of The Hero of Ages, the millennium-long struggle between the cosmic forces Preservation and Ruin has come to a close, and the Final Empire's world has been utterly transformed. Vin, the Mistborn who once toppled the Lord Ruler, spent the book hunting the Lord Ruler's hidden supply caches with her husband Elend Venture while trying to understand Ruin's growing influence—deadly mists, ash-choked skies, and koloss armies turned into weapons of mass destruction. In the climax, Vin was fatally wounded but freed by the dying Inquisitor Marsh (a former ally turned pawn of Ruin) from a hidden Hemalurgic spike disguised as her earring, and she ascended to become the new vessel of Preservation. She then used that power, combined with an echo of Ruin's own destructive nature within her, to sacrifice herself and permanently destroy Ruin at the climactic battle near the kandra homeland, the Pits of Hathsin.

Elend, who had grown into a capable and beloved king and Allomancer over the course of the trilogy, died in that same final battle, mortally wounded by Marsh, but not before ensuring that his soldiers burned away the vast stash of atium that was secretly Ruin's physical "body," preventing any chance of Ruin's return. Both Vin and Elend are dead at the book's close, their bodies found peacefully lying together in a field of flowers in the newly remade world.

Sazed, the Terris Keeper who spent much of the book in a crisis of faith after the death of his beloved Tindwyl, is revealed as the true Hero of Ages—not Vin, as most had assumed. Using the vast knowledge stored in his feruchemical copperminds along with the combined power of the now-dead Preservation and Ruin, he single-handedly reshapes the devastated, ash-covered world into a lush, colorful paradise. He then vanishes as an active character, leaving behind a comprehensive written account of the true history of Preservation and Ruin and his own transformation.

Other major figures survive and carry forward: Spook (the young former street urchin Lestibournes), who was secretly made into a full Mistborn and physically healed by Sazed as a parting gift, steps up to lead the surviving population in rebuilding society in this new world. Breeze and Ham, longtime members of Kelsier's original crew, also survive and emerge into the reformed world. TenSoon and the kandra, having accepted their prophesied role, are now committed allies of humanity rather than secretive outsiders. Sazed's note to Spook reveals that at least two metals and their Allomantic/Feruchemical alloys remain undiscovered in the world—an open thread going forward. The Final Empire, the Lord Ruler's rule, and the ancient war between Preservation and Ruin are all definitively concluded; the surviving cast now faces the very different challenge of rebuilding civilization from scratch in a transformed, green world with new metallic mysteries still unexplored.