According to the Book of Mormon, the brother of Jared is the most prominent person in the account given in the beginning (Chapters 1 - 6) of the Book of Ether. Moriancumer is the name of the place where Jared and the people he was traveling with (later to be called Jaredites) settled for a time. Some years after the publication of the Book of Mormon, Joseph Smith, Jr. mentioned that Mahonri Moriancumer was the name of the "brother of Jared."
This is information about the Brother of Jared as related in the Book of Mormon.
Tower of Babel
According to the Book of Ether, Mahonri and his brother Jared were present at the Tower of Babel. When the language of the people was confounded such that each spoke a different language, Jared asked his brother, Mahonri, to ask God not to confound their own language, that of their friends, and that of their immediate families.
Land of Moriancumer
After being granted to have their language not confounded, they asked to be led to where the Lord would have them go, hoping that it would be a choice land. They were led through the wilderness to the sea and then dwelt in tents at the seashore for four years. They named this place where they had (temporarily) settled Moriancumer. The account relates that after four years, the Lord came to Mahonri and "chastened him because he remembered not to call upon the name of the Lord."
Journey to the Promised Land
Mahonri was then instructed to build "barges" or boats. After building the boats, Mahonri worried about how the insides of the boats would be lit during their journey across the sea. The Lord told Mahonri he should figure out a way to light the boats, and so the Brother of Jared produced sixteen stones from molten rock, two for each ship, and they were white and clear, just like transparent glass. Mahonri then asked the Lord to touch each stone he had made so they would shine in the darkness.
According to the book, the Lord touched each stone with his finger. Mahonri watched as the Lord did this and fell to the ground in fear. Jesus Christ revealed to Mahonri that he had seen His spirit even as He would appear in the flesh (His premortal spirit body). Mahonri was to write down what he saw in his own language, which no one would understand, and his record would be revealed after Christ had been lifted upon the cross. (When Mosiah interpreted the account, he was instructed that these things were not to be revealed until after his people had been visited by Christ.)
The account continues to relate that the Jaredites, as they came to be known, boarded the boats, and were driven by the wind until they arrived at the "promised land."
In the Promised Land
When Jared became old, he and his brother gathered their descendants to count them: Mahonri had twenty-two descendants, and Jared, twelve.
The Jaredite people wanted a king appointed, according to the account. While Jared did not want this, he ultimately relented. After several Jaredites refused the position, Orihah accepted. Jared and his brother died during the reign of Orihah.
The name Moriancumer does appear in the Book of Ether in the Book of Mormon, as a place where the people pitched their tents. However, the name Mahonri Moriancumer is found nowhere in the text of the Book of Mormon, but he is consistently referred to as the "Brother of Jared." Joseph Smith said that the name was revealed to him, as discussed in the following quotation:
"While residing at Kirtland, Elder Reynolds Cahoon had a son born to him. One day when President Joseph Smith was passing his door he called the Prophet in and asked him to bless and name the baby. Joseph did so and gave the boy the name of Mahonri Moriancumer. When he had finished the blessing, he laid the child on the bed, and turning to Elder Cahoon he said, the name I have given your son is the name of the Brother of Jared; the Lord has just shown or revealed it to me. Elder William F. Cahoon, who was standing near heard the Prophet make this statement to his father; and this was the first time the name of the brother of Jared was known in the Church in this dispensation."
- ↑ Ether 1:33
- ↑ 2.0 2.1 Ether 2:13
- ↑ Ether 2:14
- ↑ Ether 2:16-18
- ↑ Ether 3:1
- ↑ Ether 6:24-27
- ↑ "The Jaredites," The Juvenile Instructor, [1 May 1892], p. 282 n.
- ↑ Chapter 48, Book of Mormon Student Manual, Religion 121 and 122, 1996
- ↑ "Understanding the Scriptures", "Ether 2 The Jaredites Build Barges", Book of Mormon Seminary Student Study Guide, 
- Brother of Jared by Rex C. Reeve, Jr.
- The Brother of Jared: An Expert at Learning by Henry B. Eyring
- Book of Mormon Seminary Student Study Guide (2000)
- Chapter 48, Book of Mormon Student Manual (1996)