In 811 the Byzantine Emperor Nicephorus I launched a massive offensive against Bulgaria, seized, plundered and burned down the capital Pliska but on the way back the Byzantine army was decisively defeated in the battle of the Varbitsa Pass. Nicephorus I himself was slain along with most of his troops, and his skull was lined with silver and used as a drinking cup. 

The Battle of Pliska was one of the worst defeats in Byzantine history. It deterred Byzantine rulers from sending their troops north of the Balkans for more than 150 years afterwards, which increased the influence and spread of the Bulgarians to the west and south of the Balkan Peninsula, resulting in a great territorial enlargement of the First Bulgarian Empire. This was the first time a Byzantine Emperor was killed in battle since the Battle of Adrianople in 378.