So I've had to answer this question myself. A high level 'thief' is the king of the underworld and the lord of crime. Mages can scry and fry from afar, but a thief? He mentions one day that some one needs to be 'Taken Care Of' and the next day on the other side of the realm that person winds up dead in a dumpster. They have all the wealth they need, and no kingdom dares touch them for when attacked the thief can call in favours and utterly destroy them through war and assassination.

Thieves know every thing that goes on in the realm, and are able to nudge events so that things line up as they wish. They have functionally infinite money and influence and noone knows where to find them, or sometimes even who they are. They are the Lord of Shadows.