The name of one of the five rivers of the Greek underworld and the spirits that personified it. 

It means 'Oblivion', 'Forgetfulness' or 'Concealment'. 

Those who travelled the the river or drank from it's waters forgot who they were or what they were doing. 

Seems fitting for a faction wreathed in shadow and adept at espionage. Lending itself to a vision of them as 

being so proficient, that their foes second guess whether they saw anything at all. Or maybe behaving similar

to The Silence from Doctor Who. 

Loved this name the instant I saw it, and was jealous I hadn't suggested it myself.