The Gray Fox
The Gray Fox is the mysterious leader of the Thieves Guild in the Imperial Province. Among the common population of the Empire it is often rumored that he can turn invisible at will, shrink to the size of a mouse, turn into mist and seep under locked doors, while most members of the Imperial Legion simply believe him to be a myth, nothing more than another common thief.
Among his followers, however, the rumors that flow around are quite different; they instead comment about the mysterious figure that he represents, said to have ruled the Thieves Guild for about 300 years, questioning if he could even be immortal.
Source: The Elder Scrolls IV: Oblivion