Complex sentence: When ibn Tumart died in 1130 al-Mu'min became the leader of the movement, he forged it into a powerful military force and under him the Almohads swept down from the mountains destroying the power of the faltering Almoravid dynasty by 1147.
Split into simpler sentences: When ibn Tumart died in 1130 al-Mu'min became the leader of the movement, and forged it into a powerful military force. Under him the Almohads swept down from the mountains, destroying the power of the faltering Almoravid dynasty by 1147.