The Battle of Kleidion took place in the valley between the mountains of Belasitsa and Ograzhden, near the modern Bulgarian village of Klyuch. The decisive encounter occurred on July 29 with an attack in the rear by a force under the Byzantine general Nikephoros Xiphias, who had infiltrated the Bulgarian positions. The ensuing battle was a major defeat for the Bulgarians. Bulgarian soldiers were captured and reputedly blinded by order of Basil II, who would subsequently be known as the "Bulgar-Slayer". Samuel survived the battle, but died two months later from a heart attack, reportedly brought on by the sight of his blind soldiers.

Although the engagement did not end the First Bulgarian Empire, the Battle of Kleidion reduced its ability to resist Byzantine advances, and it has been considered the pivotal encounter of the war with Byzantium.