Answer
Arrogance led to Manuel Komnenos' catastrophic defeat at Myriokephalon. His belief in his own invincibility clouded his judgment, causing him to underestimate the Seljuk Turks' strength and disregard their military tactics. Manuel's impetuosity and strategic blunders, such as dividing his army and ignoring advice, sealed the Byzantine forces' fate, resulting in a crushing loss and marking a significant turning point in Byzantine-Seljuk relations.