What it was
The Praetorian Guard was the elite corps founded by Augustus in 27 BC to protect the emperor and his family. Under Tiberius the prefect Sejanus concentrated its cohorts in the Castra Praetoria, a fortified camp at the edge of Rome, making the Guard a permanent armed presence in the capital.
Role
Praetorians escorted the emperor, guarded the palace, and helped police Rome, with higher pay and shorter service than ordinary legionaries. Their position made them kingmakers: they murdered Caligula in AD 41 and proclaimed Claudius, and in 193 they killed the emperor Pertinax and auctioned the throne to the highest bidder, Didius Julianus. Ambitious prefects such as Sejanus wielded power second only to the emperor’s.
Fate
Septimius Severus disbanded the Guard in 193 and refilled it with legionaries loyal to himself. In 312 the Praetorians backed Maxentius against Constantine at the Milvian Bridge; after his victory Constantine dissolved the Guard for good and dismantled the Castra Praetoria. The word praetorian survives as a byword for palace guards who threaten the power they serve.