Complex sentence: Peter IV besieged Tarnovo and Ivanko had to flee to the Byzantine Empire where he was made governor of Philippopolis but Peter IV himself was murdered less that a year after his brother's demise.
Split into simpler sentences: Peter IV besieged Tarnovo, and Ivanko fled to the Byzantine Empire where he was made governor of Philippopolis. Peter himself was murdered less that a year after his brother's demise.