Mistborn: Secret History is a fantasy novella by American author Brandon Sanderson, written as a companion story to the original trilogy of the Mistborn series. It was published as an ebook-only novella by Dragonsteel Entertainment LLC, a company owned by the author, on 30 January 2016.
The novella follows the story of Kelsier from the time of his death in Mistborn: The Final Empire and concludes with the events of the last book in the original trilogy, The Hero of Ages.
Sanderson published the novella, which was 12 years in the making, as an effort to help fans to tide over until the third book in The Stormlight Archive was published. The story was supposed to be a secret until published, but Amazon accidentally leaked the last few pages of The Bands of Mourning with the author's comments about the story in the novel's postscript. The author noted in a blog post that the novella contained huge spoilers for the original Mistborn trilogy as well as minor spoilers to The Bands of Mourning, warning readers that knowledge of the story's existence was in itself, a spoiler.
After being killed by the Lord Ruler, Kelsier meets Preservation as he is about to pass on to the spiritual realm. Kelsier refuses to do so and races to the Well of Ascension at the heart of the Lord Ruler's palace, thus anchoring himself to the world. Whilst trapped at the Well, he discovers another entity trapped there: Ruin, the counterpoint to Preservation, who intends to destroy the world.
When Vin accidentally frees Ruin when she gives up the power at the Well, Kelsier is released as well. With the world now under direct threat from Ruin, a dying Preservation tasks Kelsier to find help. Following Preservation's thoughts, Kelsier meets two people from different worlds, Khriss and Nazh, who point him to a covert group known as the Eyree as his only chance for aid.
Despite Ruin's dissuasion, Kelsier travels to the Eyree's fortress. He discovers that they are planning to steal Preservation's powers once he dies by using an artifact. Kelsier dupes the group by posing as Ruin and steals the artifact for himself. When he returns to the world, Preservation tells him that he cannot pass on his powers to Vin because she has been spiked by Ruin. When Preservation finally dies, Kelsier takes up the power using the stolen artifact. Trying and failing to contact Vin because of Ruin's interference and his inexperience with Preservation only exacerbating the world's troubles, Kelsier travels to Urteau and succeeds in freeing Spook from Ruin's influence.
Under Kelsier's command, Spook sends a message to Vin about her spike and Ruin's influence on her. However, Ruin intercepts and kills the messenger using Marsh, Kelsier's brother, who is now under Ruin's control. They then witness Vin's battle and subsequent defeat at the hands of Marsh and the other Steel Inquisitors. With Ruin's rage loosening his control, Marsh manages to use the intercepted message as Kelsier secretly intended and removes Vin's earring spike. Kelsier gives up Preservation's power to Vin who uses it to kill the other Steel Inquisitors but ascends before she can finish Marsh.
Kelsier realizes that Ruin's power is incomplete until he finds the Lord Ruler's hidden cache of atium. He watches the battle at the Pits of Hathsin and Elend Venture's death at Marsh's hands. With her husband's death, Vin sacrifices herself to kill Ruin. Kelsier's long-time friend, Sazed takes up the powers of both Preservation and Ruin, ascending as Harmony. Kelsier tearfully reunites with Vin and watches as she too chooses to pass on to the spiritual realm with Elend. After Harmony remakes the world, Kelsier greets Spook and recruits his help in getting him a new "string" to the physical realm.
Mistborn: Secret History was originally published 30 January 2016 by Dragonsteel Entertainment as an ebook. In November 2016, it was republished as part of Arcanum Unbounded by Tor Books.
Tor noted that the novella was an unusual one for Sanderson since it contained "less action, and a lot more time-killing while waiting for action".