Andre Norton 
Wraiths of Time 

An archaeology student is transported back to an ancient Ethiopian kingdom where, reborn as a warrior priestess, she must save their civilization from an evil as old as time   Archaeology student and expert in African history Tallahassee Mitford has been asked to authenticate some ancient artifacts. But there’s a strange energy emanating from the relics, which, according to legend, can hold the soul of an entire nation. As she examines an ankh, talisman of the Egyptian gods and the key to all life, Tallahassee hears a deafening clap of thunder, followed by total darkness. She awakens to find herself lying under a scorching desert sun, surrounded by ruins and pyramids. She has been kidnapped by the powerful spirits released from the ankh and hurled back in time to a Nubian kingdom in Meroe, a little-known nation that exists in the shadow of Egypt.   Reborn as Ashake, a magnificent warrior princess, Tallahassee must help Candace Naldamak, ancient Queen of Meroe, defeat the evil Khasti, who has found a way to pierce the walls between timelines. Ensnared in a power struggle for the throne, guided only by her knowledge of African history and her own free will, Tallahassee will endure a life-threatening trial by fire before she learns the true reason she has been summoned here.