The Byzantine Emperor Michael IV prepared a major campaign to finally defeat the Bulgarians. He gathered an elite army of 40,000 men with capable generals and moved constantly in a battle formation. There were a lot of mercenaries in the Byzantine army including Harald Hardråde with 500 Varangians. From Thessaloniki the Byzantines penetrated in Bulgaria and defeated the Bulgarians at Ostrovo in the late summer of 1041. It seems that the Varangians had a decisive role in the victory as their chief is hailed in the Norse sagas as the "devastator of Bulgaria". Though blind, Petar Delyan was in command of the army. His fate is unknown; he either perished in the battle or was captured and taken to Constantinople.

Soon the Byzantines eliminated the resistance of Delyan's remaining voivodes, Botko around Sofia and Manuil Ivats in Prilep, thus ending the Bulgarian revolt.