The Battle of Tryavna in 1190 was something that I randomly came across recently. Apparently, it was quite a blow to the Byzantines and a morale booster for the ascendant Second Bulgarian Empire. The Bulgarians managed to capture the golden helmet of the Byzantine emperors, the crown, and the Imperial Cross. I have not been able to find the number of losses but apparently the Byzantines had heavy losses during this battle.
Isaac II Angelos barely escaped with his life at this battle. What if he was killed during the charge by the Bulgarians in the Tryavna Pass?