Explanation: The Praetorian Guard was, theoretically, the personal bodyguard/secret police of Roman Emperors. They were sometimes known as the toga'd battalion (Cohors Togata) because they usually remained in the city of Rome, where they wore togas, as was traditional - considering the use of the toga as a predominantly formal garment, think of it like the secret service wearing suits.
Now, I say 'theoretically' because they were about as trustworthy as their symbol - a scorpion. No, really. Their symbol was a scorpion (it was an astrological symbol, not meant to represent treachery, but goddamn, what fateful symbolism). They were not particularly great at preventing the deaths of Emperors, and they would often extort Emperors for bonus pay or be bribed by usurpers. They may not have actually caused more assassination attempts than they prevented, but goddamn does it seem like it reading through the history of Roman palace coups.
It got bad enough that Emperors who came to power in times of turmoil pretty invariably purged the previous Praetorian Guard because the bastards couldn't be trusted. Of course, after filling the Praetorian Guard with their own loyalists, the next Emperor has a Praetorian Guard of dubious loyalty that needs to be bribed into compliance... and the cycle starts again. Stabby stabby!