I'm thinking these units would be comprised of like Nightblades and the best of the legionnaires.
Well there is/was legion of battle mages called shadow legion so maybe? I don't think there is any super special spy , badass guys only group. Blades and later Oculatus are the closest to that.
There are also Palace Guards and the like, so perhaps they usually fulfill another role, and then do what they're called upon to do.