Which Roman emperor was personally known by or written about by both Thrasyllus of Mendes and Tacitus?
Thrasyllus of Mendes:  Thrasyllus was an astrologer and a personal friend of the Roman emperor Tiberius, as mentioned in the "Annals" by Tacitus and "The Twelve Caesars" by Suetonius. Tacitus:  The surviving portions of his two major works—the "Annals" and the "Histories"—examine the reigns of the Roman emperors Tiberius, Claudius, Nero, and those who reigned in the Year of the Four Emperors (69 AD).
Tiberius