The emperor Caligula was assassinated on this day in the year 41 AD, less than 3½ years after coming to the throne upon the death of his uncle Tiberius. His reign was famously marked by insanity and bizarre, tyrannical behavior; one of the best-known episodes (and one of the few with a PG rating) was when he made his horse a member of the Senate. Since the books of Tacitus’ Annals that covered the reign of Caligula are lost, we are dependent on Suetonius (57-58) among the major historians of the early principate for an account of the murder. He begins with a series of bad omens that foretold the murder, the first of which is particularly startling.
“Futurae caedis multa prodigia exstiterunt. Olympiae simulacrum Iovis, quod dissolvi transferrique Romam placuerat, tantum cachinnum repente edidit, ut machinis labefactis opifices diffugerint; …
Capitolium Capuae Idibus Martiis de caelo tactum est, item Romae cella Palatini atriensis. Nec defuerunt qui coniectarent altero ostento periculum a custodibus domino portendi, …
Pridie quam periret, somniavit consistere se in caelo iuxta solium Iovis impulsumque ab eo dextri pedis pollice et in terras praecipitatum. …
There were many prodigies foretelling the approaching murder. The statue of Jupiter at Olympia, which he had ordered to be taken apart and brought to Rome, suddenly gave forth so loud a laugh that the scaffoldings collapsed and the workmen fled in all directions…
The Capitol at Capua was struck by lightning on the Ides of March, and also the room of the palace doorkeeper at Rome, and there were some who inferred from the latter omen that danger was threatened to the owner at the hands of his guards;
The day before he died, he dreamt that he stood in heaven beside the throne of Jupiter, and was kicked by him with the big toe of his right foot and hurled to earth. …”
Suetonius names several others, but I have noted these because it seems to be part of his presentation that Caligula had greatly offended Jupiter in various ways. For example, not only did he attempt to steal the above-named statue; he was also planning on replacing its head with the likeness of his own. Note then the password which became the signal for the assassin, a tribune of the pretorian guards named Cassius Chaerea, to strike.
“Nono Kal. Febr., hora fere septima, … in crypta, per quam transeundum erat, pueri nobiles ex Asia ad edendas in scaena operas evocati praepararentur, ut eos inspiceret hortareturque restitit, …
Duplex dehinc fama est: alii tradunt adloquenti pueros a tergo Chaeream cervicem gladio caesim graviter percussisse praemissa voce: ‘hoc age!’ Dehinc Cornelium Sabinum, alterum e coniuratis, tribunum ex adverso traiecisse pectus; alii Sabinum summota per conscios centuriones turba signum more militiae petisse et Gaio ‘Iovem’ dante Chaeream exclamasse, ‘accipe ratum!’ respicientique maxillam ictu discidisse.
(The Assassination of Caligula, by Lazzaro Baldi (1624-1703))

On the ninth day before the Kalends of February at about the seventh hour … in the covered passage through which he had to pass, some noble boys from Asia, who had been summoned to appear on the stage, were rehearsing their parts, and he stopped to watch and to encourage them …
From this point there are two versions of the story. Some say that as he was talking with the boys, Chaerea came up behind, and gave him a deep cut in the neck, having first cried, ‘Take that,’ and that then the other conspirator, the tribune Cornelius Sabinus, and faced Gaius, stabbed him in the breast. Others say that Sabinus, after getting rid of the crowd through centurions who were in the plot, asked for the watchword, as soldiers do, and that when Gaius gave him ‘Jupiter,” he cried ‘So be it,’ and as Gaius looked around, he split his jawbone with a blow of his sword.”
The cryptoporticus in which Caligula was killed was excavated by archeologists and opened to the public in 2008. It formed part of one of the various houses (domus) of the imperial complex on the Palatine hill, built by his predecessor, and thus known as the Domus Tiberiana. It is quite large, and nothing indicates the precise location of the murder, but modern visitors to the Palatine can now say that they have walked through the sight of the first imperial assassination.
(Part of the cryptoporticus of the Domus Tiberiana.)
