Moriantum is a place briefly mentioned in the Book of Mormon. In the Book of Moroni, Moroni copies a second letter that his father Mormon has written to him prior to being killed by the Lamanites. Among the things Mormon tells his son is the brutality of the Nephites in Moriantum. Having taken many of the daughters of the Lamanites prisoner, the Nephites would take their chastity and virtue. Then they would torture them until they died. Finally, they would devour their flesh. All this they would do as a token of bravery.
Moriantum is written mostly as an example by Mormon of how wicked the Nephites had become. As opposed to the typical where the Nephites were the more righteous of the two, Mormon points to this as one of the many reasons that the Nephites were far more wicked than the Lamanites.