Teancum was a Nephite military commander under Moroni during the wars against the Lamanites and Nephite dissenters. Around 67 BC he led the army Moroni sent against the rebel Morianton, defeated his force, took the survivors prisoner, and killed Morianton himself (Alma 50:35).
When the dissenter Amalickiah, then king of the Lamanites, marched to seize the land Bountiful and the land northward, Teancum and his men repulsed him and drove the Lamanites back to the seashore (Alma 51:29-32). That night Teancum and a servant stole into the camp, and Teancum entered the king’s tent and drove a javelin into Amalickiah’s heart, killing him without waking his guards (Alma 51:33-34). The next morning the Lamanites found Amalickiah dead, abandoned their plan to march into the land northward, and retreated into the city of Mulek; Amalickiah’s brother Ammoron was made king in his place (Alma 52:1-3).
To retake Mulek, Teancum took a small body of men along the seashore to draw the Lamanite army out; when they pursued him, Moroni’s force took the city (Alma 52:21-26). Afterward Teancum, by Moroni’s orders, set the Lamanite prisoners to dig a ditch and raise a wall of timber and earth around Bountiful, turning it into a stronghold guarded by the labor of the captives (Alma 53:3-5).
During Moroni’s absence to assist the chief judge Pahoran, Teancum and Lehi were given command over the remainder of the army (Alma 61:15; 62:3). Around 60 BC, after a day’s march, Teancum was angry with Ammoron, holding him and his brother Amalickiah responsible for the long war, famine, and bloodshed (Alma 62:35). He let himself over the city wall by night, found Ammoron, and cast a javelin that pierced him near the heart; but Ammoron woke his servants before he died, and they pursued and killed Teancum (Alma 62:36).
When Lehi and Moroni learned of his death they mourned; Mormon’s record calls him “a man who had fought valiantly for his country, yea, a true friend to liberty,” who had suffered many sore afflictions (Alma 62:37).