Complex sentence: John was presumably released from Antioch as he took part in the Battle of Myriokephalon which was led by Manuel I Komnenos however, they were defeated and John Doukas died during battle shortly after 17 September 1176.
Split into simpler sentences: John was presumably released from captivity in Antioch, as he took part in the Battle of Myriokephalon under Manuel I Komnenos. The Byzantines were defeated and John Doukas died during the battle, shortly after 17 September 1176.