For some reason Osorkon II chose not to (or perhaps could not) appoint one of his own sons to succeed his uncles Iuwelot and Smendes III as High Priest of Amun at Thebes, instead he turned to his cousin - Harsiese son of the late Sheshonq II (and previous High Priest of Amun). This decision to place a man who had links with the throne and High Priest hood was to have a devasting effect on Egypt - within a short while Egypt would be thrown back towards having a second king in Thebes and civil war.
Within a short while of becoming High Priest, Harsiese pronounced himself King (with titles to match). This grand gesture gave him little more power at the time, but it did stir up the Thebans with thoughts of independence from Thebes.
Harsiese did remain High Priest until his death, although in his latter days he was incapacitated by illness. His tomb was found at Medinet Habu - in which was found his skull. The bottom of his sarcophagus originally belonged to the sister of Ramesses II, Henutmire.