It's 68 CE. Following the revolt of Vindex in Gaul, the Praetorian Guard turns against Nero, who then flees Rome and commits suicide. He is succeeded by Galba, governor of Hispania Terraconensis, whose reign will last only 7 months and 7 days. The following year is the Year of Four Emperors as Otho, Vitellius, and Vespasian take the throne in succession. 

During the reigns of Claudius (41–54) and Nero (54–68), magic has begun to move more into the mainstream. Much of the old fear of it remains, and no one quite knows what role it will play in society and in the wars that will determine the fate of the Empire, but everyone’s rushing (and fighting) to figure it out.

Claudius's reign was marked by the efforts of his loyal underlings—including Galba in Hispania, Vitellius in Africa, and Vespasian in Brittania (and, later, Judaea and Aegyptus)—working to discover the secrets of native magic. Now, after Nero's death, the conflict has become as much about marshaling all this magic as it is about mustering legions. That includes wizardly artillery, necromancers with undead troops, and also monsters brought from the remotest parts of the world. The Praetorian Guard has its own secret magic, too, which enhances its role in making and breaking the would-be emperors of Rome.

Nero

From Wikipedia:

After Nero's suicide in 68, there was a widespread belief, especially in the eastern provinces, that he was not dead and somehow would return.[174] This belief came to be known as the Nero Redivivus Legend.

The legend of Nero's return lasted for hundreds of years after Nero's death. Augustine of Hippo wrote of the legend as a popular belief in 422.[175]

At least three Nero imposters emerged leading rebellions. The first, who sang and played the cithara or lyre and whose face was similar to that of the dead emperor, appeared in 69 during the reign of Vitellius.[176] After persuading some to recognize him, he was captured and executed.[176] Sometime during the reign of Titus (79–81), another impostor appeared in Asia and sang to the accompaniment of the lyre and looked like Nero but he, too, was killed.[177] Twenty years after Nero's death, during the reign of Domitian, there was a third pretender. He was supported by the Parthians, who only reluctantly gave him up,[178] and the matter almost came to war.[111]

I'm pretty sure Nero became a vampire. And none of those three "imposters" were properly staked and decapitated.