Caligula, Life of a Roman Emperor

Suetonius' most important surviving work is known as the De Vita Caesarum, a set of twelve biographies of the successive Roman rulers. The emperor Caligula was one of the strangest and cruelest rulers that ever lived, and most of what is known about his reign comes from Suetonius. The first six months of his reign appear to have been moderate and successful; but after that, he fell into a spiral of sadism, sexual perversity, and plain insanity that made him one of the most hated tyrants of all time.
His biography gathers many extravagant anecdotes, from naming his horse consul to calling people to his palace in the middle of the night just to play a strange game of fright with them. He was eventually assassinated, but not before many strange omens predicted the event, according to Suetonius.