Mistborn: The Secret History is a companion story to the main Mistborn trilogy. It contains MANY SPOILERS for the novels: Ash and Steel, Well of Ascension, Hero of Ages, and also very minor spoilers for Bracelets of Sorrow. The story is entirely built on the characters, events and world order of the Mistborn series, and it should not be read by anyone unfamiliar with the first trilogy. Overall, this is not a good starting point for anyone looking to embark on a journey through the world of Mistborn. But if you've read the entire first trilogy and remember the characters and storylines well, then you're fine. There's no way to say more about this story without giving away too much. In fact, the very fact of its existence is already a spoiler. There are always secrets.