The Quest — Wilbur Smith

A CIVILIZATION AT THE BRINK OF RUIN
Egypt has been struck by a series of terrible plagues, killing its crops and crippling its people. Then the ultimate disaster befalls the kingdom: the Nile fails. The waters that nourish and sustain the land dry up. Something catastrophic is taking place in the distant and totally unexplored depths of Africa, from where the mighty river springs...

A PHARAOH'S LAST HOPE FOR HIS EMPIRE
In desperation, the pharaoh sends for the warlock Taita, wise in the lore of the gods and a master of magic and the supernatural. He is the only man who might be able to find his way through the hazardous territory to the source of the Nile and discover the cause of all their woes.

A WARLOCK FACES THE FINAL FRONTIER
Now it's up to Taita to save Egypt before the end of civilization draws near. But not even he could have imagined what terrible enemy waits in ambush in those dark lands at the end of their world...