Alma 17:20

And as Ammon entered the land of Ishmael, the Lamanites took him and bound him, as was their custom, to bind all the Nephites which fell into their hands, and carry them before the king; and thus it was left to the pleasure of the king to slay them, or to retain them in captivity, or to cast them into prison, or to cast them out of his land, according to his will and pleasure;

When Ammon entered the land of Ishmael, the Lamanites took him and tied him up. This was their custom. They tied up all the Nephites they caught and took them to the king. The king could decide to kill them, keep them as prisoners, put them in jail, or send them away. He could do whatever he wanted.

As soon as Ammon entered the land of Ishmael, the Lamanites grabbed him and tied him up. That’s what they always did with Nephites who fell into their hands—they brought them before the king. The king could do whatever he wanted: kill them, throw them in prison, keep them as captives, or drive them out of the land.