Complex sentence: Hearing from Byzantine agents (showing him letters) that king Khosrau II, dissatisfied with his failure to capture Constantinople, is planning to have him executed, he surrenders to Heraclius, refusing to join the Byzantine army against his ungrateful sovereign.
Split into simpler sentences: Hearing from Byzantine agents (showing him letters) that king Khosrau II, dissatisfied with his failure to capture Constantinople, is planning to have him executed. Surrenders to Heraclius, refusing to join the Byzantine army against his ungrateful sovereign.