The Seventy Lost Djinn of Akhenaten were the djinn bound as slaves by Akhenaten during his reign. They were buried in a container in his tomb. The location of the lost djinn was hidden but an earthquake revealed a clue to where it was.
Whoever freed the djinn had the power to dictate which side they serve- good or evil. The plot of the first book centered around the twins and Nimrod battling the forces of evil for control of the djinn, who would greatly upset the equilibrium of luck. If they fell into the wrong hands, they could cause a catastrophe.
Philippa Gaunt correctly guessed that they were hidden within the Egyptian scepters in the British Museum through Iblis's drawings. The djinn themselves were small, bald headed, shadowed-eyed men dressed in the white robes of Egyptian priests.