Sethos is the nom de guerre of the shadowy "Master Criminal" in the Amelia Peabody series of historical mystery novels.
He is first encountered in The Mummy Case, as the mastermind of an organized gang of thieves attempting to steal antiquities from Dahshoor, in which he is partially foiled by Amelia Peabody and her husband, Radcliffe Emerson.
In Lion in the Valley, his nom de guerre is revealed, and he is thus described by an Englishman who hears of him during his brief foray into slum life: Amelia becomes obsessed with the idea of apprehending the Master Criminal and putting a stop to his activities.
It is revealed in He Shall Thunder in the Sky that "Sethos" is actually Emerson and Walter's half-brother, the illegitimate son of their father and his mistress, who had been their mother's best friend.
In Lion in the Valley he admits to appearing to the Emersons in three vastly different roles: a young, vacuous English nobleman, a middle-aged private investigator, and an elderly and obnoxious American female tourist.