Niccolò Machiavelli only mentions Lucius Aelius Sejanus (aka Sejanus; 20 BCE – CE 31) once in his works: in a single paragraph found in his Discourses, Book III, Chapter VI—Of Conspiracies. Even then, Machiavelli only mentions him in passing in a very short list of conspirators (trans. Marriott, emphasis added):
We see, however, that the great majority of conspirators have been persons of position and the familiars of their prince, and that their plots have been as often the consequence of excessive indulgence as of excessive injury; as when Perennius conspired against Commodus, Plautianus against Severus, and Sejanus against Tiberius; all of whom had been raised by their masters to such wealth, honours, and dignities, that nothing seemed wanting to their authority save the imperial name. That they might not lack this also, they fell to conspiring against their prince; but in every instance their conspiracies had the end which their ingratitude deserved.
This oversight surprises me because the life and actions of Sejanus seem a rich topic for Machiavelli to have elaborated on, especially in his chapter on conspiracies. Yet Machiavelli has no more to say about the second-in-command under the Roman emperor Tiberius who came within throwing distance of overthrowing the emperor. Sejanus was a master conspirator until, of course, his final, failed gamble. But he came so very close to winning.