Zemnarihah was the leader of the Gadianton robbers after Giddianhi, who had been overtaken and slain while fleeing a failed assault on the Nephites (3 Nephi 4:14). In the twenty and first year (AD 21), Zemnarihah directed the robbers to lay siege to the gathered Nephites on every side, intending to cut them off from their lands and outward privileges and force them to surrender (3 Nephi 4:16-17).
The siege failed because the Nephites had stored up much provision while the robbers had only the meat they hunted, and the wild game grew scarce until the robbers were near starving. Nephite forces marched out day and night, cutting the robbers down by the thousands, so Zemnarihah’s people wished to abandon the siege. He ordered them to withdraw and march into the furthermost parts of the land northward (3 Nephi 4:22-23).
Gidgiddoni, aware of their weakness and their plan to retreat, sent his armies out by night to get beyond the robbers and cut off their retreat, meeting them front and rear. Many thousands surrendered and the rest were slain. Zemnarihah was taken and hanged upon the top of a tree until he was dead; the Nephites then felled the tree to the earth and cried for the Lord to preserve his people against all who seek to slay them through power and secret combinations (3 Nephi 4:28-29).