Mosiah 19:15

therefore the Lamanites did spare their lives, and took them captives, and carried them back to the land of Nephi, and granted unto them that they might possess the land, under the conditions that they would deliver up the king Noah into the hands of the Lamanites, and deliver up their property, even one half of all they possessed; one half of their gold, and their silver, and all their precious things; and thus they should pay tribute to the king of the Lamanites, from year to year.

So the Lamanites did not kill them. They took them as prisoners back to the land of Nephi. They let them live there if they would give up King Noah. They also had to give half of everything they owned. This meant half their gold, silver, and valuable things. They had to pay this amount every year to the Lamanite king.

So the Lamanites spared them and took them captive. They marched them back to the land of Nephi and let them stay there under strict conditions: they had to hand over King Noah, surrender half of everything they owned—half their gold, silver, and precious things—and pay annual tribute to the Lamanite king.