See Mosiah 20:3-5. Amulon was one of King Noah’s priests who undoubtedly took an active part in the martyrdom of Abinadi. When Noah was burned to death by his enraged subjects, Amulon was among those who fled into the wilderness.
Having abandoned their wives and children for fear of their own safety, these ignoble priests then kidnapped some Lamanite maidens, whom they took with them in their wilderness wanderings.
The Lamanites, supposing that their daughters had been taken by those of the city of Nephi, then attacked the Nephites, which resulted in the loss of many lives (see Mosiah 20).
Later these priests made an alliance with the Lamanites and were made rulers over Alma and those of the city of Helam. Their brutality and injustice caused Alma and his people to implore the Lord to deliver them.
Ashamed of the actions of their forbears, the posterity of Amulon and his fellow priests now chose to be known no longer by the names of their fathers. Thus they took upon themselves the name of Nephi.