The Conspiracy of Evil by Christian Jacq
Despite the best efforts of the Pharaoh Senusret and his generals, the beautiful acacia tree of Abydos is dying. But the beautiful priestess Isis has an idea: to save the 'Tree of Life', a new pyramid must be built in honour of Osiris. Senusret agrees to the plan - but what the pharaoh doesn't know is that a triple conspiracy has been hatched against him. On the one hand, the Herald, the bearded devil who preaches to the desert tribes, has decided to seize power by having the pharaoh murdered. On the other, within Senusret's own court, an ambitious, thieving traitor is plotting to get rid of his master. Finally, Iker the scribe, deceived by appearances, believes that the time has come to settle his scores. Never has the pharaoh seemed so alone or so vulnerable. And never before has the future of Egypt been in such great peril.