By about AD 401 to 421, the Lamanites have destroyed most of the Nephites. While in hiding, Moroni finishes and preserves his father Mormon's record of the people. Based on actual events as recorded in the Book of Mormon: Another Testament of Jesus Christ.
After the great and tremendous battle at Cumorah, behold, the Nephites who had escaped into the country southward were hunted by the Lamanites, until they were all destroyed.
And my father also was killed by them, and I even remain alone to write the sad tale of the destruction of my people.
And whether they will slay me, I know not.
But the eternal purposes of the Lord shall roll on, until all His promises shall be fulfilled.
For out of the earth shall these covenants and prophecies come, by the hand of the Lord; and it shall come even as if one should speak from the dead. The Lord hath shown unto me great and marvelous things concerning that which must shortly come, at that day when these things shall come forth among you.
I speak unto you as if ye were present, and yet ye are not. But behold, Jesus Christ hath shown you unto me, and I know your doing.
Condemn me not because of mine imperfection, but rather give thanks unto God that He hath made manifest unto you our imperfections, that ye may learn to be more wise than we have been.
I have not as yet perished; and I make not myself known to the Lamanites, because they put to death every Nephite that will not deny the Christ.
And I will not deny the Christ; wherefore, I wander whithersoever I can for the safety of mine own life.
Now I write unto my brethren, the Lamanites; and I would that they should know that more than four hundred and twenty years have passed away since the sign was given of the coming of Christ.
And I seal up these records, after I have spoken a few words by way of exhortation unto you. Behold, I would exhort you that when ye shall read these things, if it be wisdom in God that ye should read them, that ye would remember how merciful the Lord hath been unto the children of men, from the creation of Adam even down until the time that ye shall receive these things, and ponder it in your hearts. And when ye receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, He will manifest the truth of it unto you, by the power of the Holy Ghost. And by the power of the Holy Ghost ye may know the truth of all things.
Wherefore, deny not the power of God; for He worketh according to the faith of the children of men, the same today and tomorrow, and forever. And God shall show unto you, that that which I have written is true. Come unto Christ, and be perfected in Him, and deny yourselves of all ungodliness, and love God with all your might, mind and strength, that by His grace ye may be perfect in Christ.
Wherefore, this book is an abridgment of the record of the people of Nephi, and also of the Lamanites--Written to the Lamanites, who are a remnant of the house of Israel; and also to Jew and Gentile--Written by way of commandment, and also by the spirit of prophecy and of revelation--Written and sealed up, and hid up unto the Lord--To come forth by the gift and power of God in due time by way of the Gentile, to show unto the remnant of the house of Israel what great things the Lord hath done for their fathers; and that they may know the covenants of the Lord, that they are not cast off forever--And also to the convincing of the Jew and Gentile that Jesus is the Christ, the Eternal God.