Complex sentence: Kitchen 1996, § 333 During his reign he had to face--and ultimately submit to--the Kushite king Piye and his conquest of Egypt, as well as dealing with the threatening Neo-Assyrian Empire outside his eastern borders.
Split into simpler sentences: Kitchen 1996, § 333 During his reign, he had to face the power, and ultimately submit himself--to the Kushite king Piye during Piye's conquest of Egypt. Osorkon IV also had to deal with the the threatening Neo-Assyrian Empire outside his eastern borders.