Mistborn: Secret History builds upon the characterization, events, and worldbuilding of the original trilogy. Reading it without that background will be a confusing process at best.
In short, this isn’t the place to start your journey into Mistborn. (Though if you have read the trilogy—but it has been a while—you should be just fine, so long as you remember the characters and the general plot of the books.)
Saying anything more here risks revealing too much. Even knowledge of this story’s existence is, in a way, a spoiler.