The Amulonites are descendants of the wicked priests of King Noah as discussed in Mosiah 19.10-12. These wicked priests, in about 142 B.C., kidnap the daughters of the Lamanites and create new families. Sometime between 142-122 B.C. these families are integrated into Lamanite society, and Mormon emphasized that these Amulonites had great hatred for the Nephite culture. During the missionary journeys of Alma and the sons of Mosiah, these Amulonite people persecute the missionaries and cause all kinds of problems, even to the point of killing converted Lamanites (Alma 24.28; 25.5).
Mormon explains how the Amulonites begin to lose political influence, even to the point of eventually being hunted and destroyed by the Lamanites:
And the remainder, having fled into the east wilderness, and having usurped the power and aauthority over the Lamanites, caused that many of the Lamanites should perish by fire because of their belief—
For many of them, after having suffered much loss and so many afflictions, began to be stirred up in remembrance of the words which Aaron and his brethren had preached to them in their land; therefore they began to disbelieve the traditions of their fathers, and to believe in the Lord, and that he gave great power unto the Nephites; and thus there were many of them converted in the wilderness.
And it came to pass that those rulers who were the remnant of the children of Amulon caused that they should be put to death, yea, all those that believed in these things.
Now this martyrdom caused that many of their brethren should be stirred up to anger; and there began to be contention in the wilderness; and the Lamanites began to hunt the seed of Amulon and his brethren and began to slay them; and they fled into the east wilderness. And behold they are hunted at this day by the Lamanites (Alma 25.5-9).
This eventual destruction of the Amulonites is a longer, drawn out narrative that is essentially demonstrating King Noah’s demise in a very similar way. It just took a longer period of time. Basically Abinadi’s prophecy (Mosiah 17.15-19) was fulfilled, in fast fashion with Noah and his death, and over time with the wicked priests of Noah. Mormon is making sure that we do not miss his point: the words of the prophets are vindicated.