Areani (Historical version):
The Areani were agents in Imperial Roman military units based in Roman Britain during the later part of the Roman occupation of the island. They were used both as scouts for the legions and as undercover spies. Many of them were mercenaries. At the time of the Great Conspiracy, which they helped instigate, many of them were sailors. Due to their participation in the Conspiracy, Count Theodosius disbanded them.

They are sometimes (inaccurately) called Arcani, for example in the video game Rome Total War, in which they are portrayed as ninja-esque soldiers, dressed and masked in black armour.

Arcani (Rome Total War version):
They are a shadowy part of the Roman army organised in small groups and competent in camouflage and deception. The Arcani exist almost as a secret society whose very name should not be spoken. The armour they wear is finely crafted so that while it offers great protection, it will not slow or tire the fantastically fit arcani troops. Their face masks mean opposing troops may even flee in terror when surprised by a group of these blood-thirsty killers!

Praetorian (wikipedia):
The Praetorian Guard (Latin: PRÆTORIANI) was a special force of guards used by Roman Emperors. Before being appropriated for the use of the Emperors' personal guards, the title was used for the guards of Roman generals, at least since the rise to prominence of the Scipio family around 275 BC. Constantine I dissolved the Guard in the fourth century AD.

The praetorian guard was the body guard of the emperor: its power came from being so physically close to him and thus, in some cases, able to influence him or even to kill him. The commander of the praetorians (called prefectus praetorio) often took advantage of his position to control weaker emperors or to replace them (sometimes, becoming himself an emperor).

However, even though the praetorian guard is traditionally depicted as an haven of intrigue, conspiracy and murder, it was also a strong military unit that fought bravely for over two centuries, and had a somehow positive influence over the Empire by supporting strong and popular emperors; only during the crisis of the third century, it turned into the mercenary and meddling force as we infamously describe it today.

My Version:
The Emperor smell a plot of his murder by his elite guard. He summoned the ruthless and fearsome killer...the Arcani, to dealt with this conspiracy.