The Angel Moroni appeared to Joseph Smith and told him to reestablish God's Church on Earth.
I've always found this and the golden tablets to be of particular interest along with the doctrine of blood atonement, invoked by Brigham Young to justify the Mountain Meadows Massacre. (Typified here as the first 9/11.)
You have vastly oversimplified a great tragedy for which Brigham Young was not directly responsible. You might check out this thread:
Mountain Meadows Massacre Discussion
Millennial Star (weblog)
http://www.millennialstar.org/index.php/2006/09/23/p1797
It has an up-to-the minute summary of work of leading historians on the subject