Petar's mother was expelled while pregnant from Samuel's court before the accession of Gavril Radomir, but given Delyan's subsequent career, it is likely that he had been born and remained in Bulgaria with his father.

Delyan was proclaimed emperor (Tsar) of Bulgaria there and took the name Petar II after being raised atop a shield by leaders of the resistance.

Petar II Delyan took Niš and Skopje, first co-opting and then eliminating another potential leader in the person of one Tihomir, who had led a rebellion in the region of Dyrrhachium.

Since Alusian was of the blood of Samuel of Bulgaria, he was quickly proclaimed emperor in Petar II's place by his troops, but he conspired to defect to the Byzantines.

As the Bulgarian and Byzantine troops were preparing for battle, Alusian deserted to the enemy and headed for Constantinople, where his possessions and lands were restored to him, and he was rewarded with the high court rank of magistros.

Meanwhile, though blind, Petar II Delyan resumed command of the Bulgarian forces, but the Byzantine Emperor Michael IV determined to take advantage of the situation and advanced against them.

In an obscure battle of Ostrovo, the Byzantines defeated the Bulgarian troops and Petar II Delyan was captured and taken to Constantinople, where he was perhaps executed.

Norse sagas refer to the participation of the future Norwegian King Harald Hardrada, who allegedly cut down Petar II in the field of battle as a member of the Varangian Guard.