Complex sentence: The sultan slaughtered civilians on his way to Constantinople, which he quickly besieged, Duke Jakša hearing of this, wanted to return but was warned that if they would not continue, the Sultan would destroy the Serbs.
Split into simpler sentences: The sultan slaughtered civilians on his way to Constantinople, which he quickly besieged. Jakša, hearing of this, wanted to return but was warned that if they would not continue, the sultan would destroy the Serbs.