The presence of a chapter header suggests that there is some change in a source. In the book of Alma, we saw that Mormon used a personal record that Alma wrote which was different from the large plates, which he also kept. On that personal record, Alma recorded texts that came from the sons of Mosiah.
In this case, we have had what must have been a large-plate source used for much of the book of Helaman, as evidenced by the consistent use of years to divide up the narrative. Now, however, we have the second addition to that record. The first began in Chapter 7, which was marked as “the prophecy of Nephi, the son of Helaman.”
This entry is about Samuel, the Lamanite. It may not have been on the large plates, but was certainly written down in some record, perhaps the personal record that Nephi kept, which will be the most probable source for much of 3 Nephi. Mormon has set up the condition where we have seen the Nephites fall away from faithfulness and the Lamanites have become more righteous. They were converted through Nephi and Lehi’s efforts, which were reported before the disheartening return to the Nephite people, whose repentance was so hard to achieve, and so short lived.
Into that condition of general apostasy comes a second prophet, yet another witness. This time, it is a prophet from the Lamanites. Mormon continues to show that when the Lamanites are converted, they may become even more righteous than the Nephites.