Spqr V - John Maddox Roberts

Every culture has its wild entertainment, but the Roman Saturnalia is looked upon as the granddaddy of them all.

Roberts's vivid fictional account of the annual event takes place in first century Rome is a historically correct picture of excess, with the city's ordinary life shut down, all rules and most laws inoperative, and even slaves permitted to celebrate without restraint. At the same time, the noted family of Decius Caecilius Metellus the Younger have reluctantly summoned the young playboy, banished to the island of Rhodes for his wild ways, to seek his help in solving a crime that touches them directly and dangerously.

They need him to use his investigative talents to look into the poison death of a politically powerful relative and the suspicion that it is the man's wife who administered the dose. Decius, convinced she is innocent (of this murder, at any rate) looks further for the killer---a quest that leads him to a secret, banned cult of witchcraft being practiced by highborn Romans of both sexes. With the witches at his heels, Decius plunges into the maelstrom of Rome at the height of Saturnalia, and barely escapes with his life.