The Well of Ascension

The siege of Luthadel comes to a head when Straff Venture withdraws his army, intending to let the koloss horde (nominally led by Jastes, who has lost control of them) smash the city before he swoops in to "save" it. Jastes flees the rampaging koloss and is killed by Elend. Vin races back to the city just in time to stop the koloss from massacring the remaining civilians, but Dockson, Tindwyl, and Clubs are killed in the chaos. Vin discovers she can use Allomancy to command the koloss directly, and she turns the horde against Straff's army, killing Straff and his generals herself. With the koloss threat neutralized and Straff dead, Cett chooses to ally with Luthadel rather than continue his own siege. Vin then forces Cett, the newly elected Lord Penrod, and Straff's surviving general to swear fealty to Elend, whom she publicly names emperor.

Alongside the political and military crisis, Vin's uneasy nighttime alliance with Zane, Elend's half-brother, comes to a violent end. Zane has been pushing Vin to abandon Elend and the city, and after he attempts to kill her, he reveals that the real OreSeur is long dead and that the kandra she has been living with is actually TenSoon, Zane's own spy. TenSoon, however, has come to care for Vin and helps her kill Zane before departing for the kandra homeland. Freed from Zane's influence, Vin commits fully to Elend, accepting the marriage proposal he had made earlier.

Vin ultimately realizes that the legendary Well of Ascension is not hidden in Terris at all, but lies beneath Luthadel itself, having been relocated there by the Lord Ruler centuries earlier. She, Elend, and Spook find the hidden entrance beneath the Lord Ruler's palace. There, a mysterious figure formed of mist stabs Elend. Tempted to use the Well's power to save him and remake the world as she sees fit, Vin instead follows instructions Sazed had smuggled to her and releases the Well's power rather than claiming it for herself. The instant she does, a vast entity that had been trapped within the Well breaks free, declaring its own liberation. The mist figure then guides Vin to a bead of metal that she feeds to the dying Elend, transforming him into a Mistborn and saving his life through his own new ability to burn pewter.

In the aftermath, Sazed returns to the ruined Ministry stronghold where he first found the ancient inscription about the Hero of Ages, and discovers that the words had been subtly altered from what he'd copied — proof that something had been manipulating the transcriptions in his metalminds all along. He realizes the entire prophecy pointing to a "Hero of Ages" was a deception engineered to trick someone into releasing the imprisoned power at the Well. Devastated that his lifelong faith and scholarship were used as a tool for this manipulation, Sazed loses his faith. Meanwhile, Marsh, who had briefly attacked Sazed near the Well under compulsion from the Ministry, vanishes again, his loyalties left uncertain.

As Hero of Ages opens, the political map has been reshaped by the end of the siege of Luthadel. Elend Venture has been formally crowned emperor, with Ashweather Cett, Lord Penrod, and Straff Venture's surviving general all having sworn fealty to him as subordinate kings — a fragile new empire built from former rivals. Straff Venture is dead, killed by Vin, and Jastes, the former friend who led the koloss army, was killed by Elend after losing control of his forces. The koloss themselves are now nominally controllable through Allomancy, a power Vin discovered during the battle, and they remain a looming, unstable military asset/threat.

Vin and Elend are now a committed couple, having gotten past the rupture caused by Zane, Elend's half-brother and a rival Mistborn who tried to manipulate Vin into abandoning the city and killing indiscriminately. Zane is dead, killed by Vin with help from TenSoon, a kandra who had secretly replaced Vin's companion OreSeur (the real OreSeur was killed earlier by Zane's people). TenSoon has since departed for the kandra homeland, so Vin no longer has a kandra companion. Elend, previously non-Allomantic, was turned into a Mistborn near the very end of the book when a mist-spirit guided Vin to feed him a metal bead after he was stabbed — he is now a novice Mistborn learning his new powers.

The central discovery of the book is that the legendary Well of Ascension was hidden beneath Luthadel itself all along, moved there by the Lord Ruler after his own ascension centuries before. Vin found it, and — resisting the temptation to seize its power for herself to save Elend or remake the world — she released the power for "the good of the world," per instructions Sazed had prepared for her. The instant she did, an ancient and enormously powerful entity that had been imprisoned within the Well broke free, declaring itself liberated. Its nature and intentions are unknown, but it is clearly a major new threat going forward.

Sazed, the Terris Keeper, spent much of the book researching an ancient prophecy about a "Hero of Ages," coming to believe Vin might be that figure. After the Well's release, he discovered that the ancient text he'd been relying on had been deliberately altered — both in a metal engraving he'd copied and even within his own transcribed metalminds — meaning the entire Hero of Ages prophecy was likely a manipulation designed to trick someone into freeing the entity from the Well. This revelation shattered Sazed's religious faith, leaving him in a state of crisis as the book ends. Meanwhile, Marsh — Kelsier's brother, now a Steel Inquisitor working for the Ministry — briefly attacked Sazed near the Well under some compulsion before escaping again; his true loyalties and fate remain an open question. Vin, Elend, and the surviving crew (including Ham and Spook) now face the task of governing a new empire while an unknown entity, freed from ancient imprisonment, is loose in the world — with the ominous strangeness of the mists (noted throughout the book) still unexplained.