Only twenty-four Nephites survived the final battle with the Lamanites at Cumorah around AD 385. Moroni was one of the twenty-four. He later reported about the conditions that followed.
And now it came to pass that 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. But behold, they are gone, and I fulfil the commandment of my father. And whether they will slay me, I know not....
And behold, the Lamanites have hunted my people, the Nephites, down from city to city and from place to place, even until they are no more; and great has been their fall; yea, great and marvelous is the destruction of my people, the Nephites.