Messalina (c. 20–48 AD)

She became empress in her early twenties, almost by accident. When the Praetorian Guard proclaimed her overlooked husband Claudius emperor after Caligula's assassination, Valeria Messalina found herself at the center of Roman power with little preparation and enormous exposure. What she did with that position depends almost entirely on who you trust.

The ancient sources are unanimous in their verdict: sexually voracious, ruthlessly manipulative, a corrupting force on a weak emperor. But Tacitus drew heavily on the memoirs of her successor Agrippina the Younger - hardly a neutral source - and the most lurid stories strain credulity on their own terms. What is better established is that she wielded real influence, eliminated enemies with effectiveness, and accumulated power in ways that eventually made her dangerous to those closest to her.

Her downfall came in 48 AD when, while Claudius was away, she conducted a public marriage ceremony with the senator Gaius Silius. Whether conspiracy or catastrophic miscalculation, historians still argue. Claudius's freedman Narcissus moved against her before Claudius could be persuaded to show mercy. She was twenty-eight and executed in the Gardens of Lucullus - gardens she had coveted, obtained through the destruction of their previous owner, and made her own. History has a particular taste for that kind of irony.