In 1017 Basil II invaded Bulgaria with a large army including Rus' mercenaries. His objective was the town of Kastoria which controlled the road between Thessaly and the coast of modern Albania. Basil took the small fortress of Setina located between Ostrovo and Bitola to the south of the river Cherna. The Bulgarians under the command of Ivan Vladislav marched to the Byzantine camp. Basil II sent strong units under Diogenes to repulse the Bulgarians but the troops of the Byzantine commander were ambushed and cornered. To save Diogenes, the 60-year-old Byzantine Emperor moved on with the rest of his army. When the Bulgarians understood that they retreated chased by Diogenes. According to the Byzantine historian John Skylitzes the Bulgarians had many casualties and 200 were taken prisoners.