Avraham Gileadi notes that ancient Near Eastern suzerain-vassal covenant relationships might shed some light on multiple statements regarding the safety of the sons of Mosiah, and Ammon in particular as they preached to the Lamanites. According to these suzerain-vassal covenant agreements, when a vassal or servant demonstrated exceeding loyalty to a suzerain or lord, the suzerain was bound by the covenant, and gave a covenant promise to protect the vassal or his ruling heir.
In the book of Alma, we find that after preaching to the Nephites, the sons of King Mosiah desired to take the gospel to the Lamanites. No one had succeeded in doing so up to that point, and it was a dangerous mission at best. Being loyal sons, they sought their father's permission, but King Mosiah deferred granting their request (Mosiah 28:1-5). As they persisted in asking their father, Mosiah, at last, inquired of the Lord. The Lord answered, "Let them go up, for many shall believe on their words . . . and I will deliver thy sons out of the hands of the Lamanites" (Mosiah 28:5-7). Given such an assurance of divine protection, Mosiah let them go (Mosiah 28:8).
During these missionary labors we find a number of specific statements which reinforce the Lord's covenant "suzerain" promise of protection. First, Ammon defended the king's flocks against marauders who "could not hit him with their stones" (Alma 17:35). Ammon's fellow servants reported to King Lamoni, "We know he cannot be slain" (Alma 18:3). Second, after Ammon preached to Lamoni, both the king and queen and their servants were overpowered by the Spirit. Upon seeing their king prostrate, the brother of one whom Ammon had slain protecting the king's flocks raised his sword to kill Ammon. Instead, he himself fell dead (Alma 19:22). Mormon, in abridging the account, thus affirms, "Now we see that Ammon could not be slain" (Alma 19:23). Additionally, Ammon, "being the chief among them," was able to mediate the release of his brethren from prison (Alma 20:2, 28-30).
Thus, this suzerain-vassal covenant relationship helps one see the covenant types and shadows of the Lord's relationship with his servants as narrated in the Book of Mormon. [Avraham Gileadi, The Last Days: Types and Shadows from the Bible and the Book of Mormon, pp. 181, 224-225]