Complex sentence: Afonso was assisted in his attempt to become king by his mother, who kept news of João's death a secret, and arranged for Afonso to return to the capital city of Mbanza Kongo and gather followers.
Split into simpler sentences: Afonso was assisted in his attempt to become king by his mother, who kept news of João's death a secret. This gave Afonso time to return to the capital city of Mbanza Kongo and gather followers.