Complex sentence: Accusing the magnates of treachery against "foremostly God" and himself, Tvrtko fled Bosnia with his mother, and was replaced with his younger brother, who had hitherto functioned as "junior ban".
Split into simpler sentences: Accusing the magnates of treachery against "foremostly God" and himself, Tvrtko fled Bosnia with his mother. He was replaced with his younger brother, who had hitherto functioned as "junior ban".