Complex sentence: After the death of Ivan Asen II, there was some unrest in Serbia; the Mongols invaded the Balkans, including Serbia, and devastated the lands, and the Serbian nobility rose up against Vladislav.
Split into simpler sentences: After the death of Ivan Asen II, there was unrest in Serbia. The Mongols invaded and devastated the Balkans, including Serbia; the Serbian nobility rose up against Vladislav.