It is impossible to call the period of the Nubian pharaohs an unknown chapter from Egyptian history; I guess every three years, a book is written about them (e.g., R.G. Morkot, The Black Pharaohs. Egypt’s Nubian Rulers ), and I have seen two major exhibitions about them in my small country.
Perhaps, the importance of the Twenty-fifth Dynasty is overestimated. By the mid-eighth century, it seemed that divided Egypt was slowly reuniting again, although it was unclear who was to do this. In the north, Tefnakht of Sais (Twenty-fourth Dynasty) was increasingly powerful, and from the south, king Kashta seized control over Thebes. The collision between the two powers is documented on Piye’s Victory Stela: when Tefnakht started to gain control of Upper Egypt, Piye of Nubia defeated Tefnakht’s armies, captured Memphis, and subdued the princes of Lower Egypt.
His successors, Shabaqo, Shebitqo, and Taharqo (photo), ruled over both kingdoms, and found themselves entangled in the conflict between Assyria and the city states of Palestina. In 701, Egyptian armies were defeated, but they prevented the Assyrian king Sennacherib from seizing all of Palestine; as is well-known, Jerusalem kept its independence.
Thirty years later, Sennacherib’s successor Esarhaddon invaded Egypt and expelled the Nubians in 671. King Taharqo managed to get back, but was defeated again by Esarhaddon’s successor Aššurbanipal. When he found himself caught in a civil war, he recalled his troops from Egypt, and the country was reunited by Psamettichus, a descendant of Tefnakht, who founded the Twenty-sixth Dynasty.