Just days after Lehi died, Nephi acted as ruler and teacher, admonishing Laman, Lemuel, and the sons of Ishmael. But they became angry with him, and as they complained amongst themselves their anger grew. They did not want Nephi to rule over them, and they plotted to kill him.
Jesus warned Nephi of their plot and told him to flee to the wilderness with all who would follow. So Nephi, Zoram, Sam, Joseph, and Jacob and their families took their tents and whatever else they could carry and departed into the wilderness, walking for days.
They settled in a place that they named Nephi, and these people and their descendants became known as the Nephites. They kept Jesus's commandments and were industrious and extremely prosperous. They built buildings and a temple. They worked with wood and metals. They had abundant crops and flocks and herds of every kind of animal. The Nephites became numerous in the land and they were white, delightful, and extremely attractive.
Nephi brought the Liahona, the brass plates containing the record of the Jews, and the sword of Laban with them. He made copies of Laban's sword to arm his people in case his brothers' people—now known as the Lamanites—tried to destroy them, for Nephi knew that they hated his people.
Because they would not listen to the words of Jesus, the Lamanites were cut off from the presence of Jesus. Jesus caused a sore curse to come upon them because of their wickedness. He caused a skin of blackness to come upon them so that they would be loathsome and not enticing to the Nephites.
Some people have thought that the dark skin was itself the curse, but this is mistaken. The curse was being cut off from the presence of Jesus. The dark skin was the sign of the curse. Jesus also said that the children of any couple where one parent was Nephite and the other a Lamanite would also be cursed.
And Jesus told Nephi that the Lamanites would be an instrument of punishment to his people. To the extent that the Nephites failed to remember Jesus, the Lamanites would be used to punish them even to the point of complete destruction.