The scripture says it was New Years Day of 66 B.C. when the guards in the tent of Amalickiah awakened to find their king with a javelin in his heart. There must have been pandemonium in the Lamanite camp. A portrait of Teancum poised to slay Amalickiah will be found on page 93 of volume three of Treasures from the Book of Mormon.
The massive retreat of the Lamanite army to the city of Mulek meant they had fled to the nearest fortified city along the east seacoast to reconnoiter and choose a new leader.