CSM has always been a part of Mulders life whether he knew it or not. Him and Mulders father worked together in the earliest day of the secret government project. In fact, CSM may be Mulders father. In a 4th season episode "Samantha" tells Mulder that CSM is her father. We don't know if this was the real Samantha or not. But in the 7th season episode, The 6th Extinction, CSM tells Mulder that he is his father but Mulder was dreaming at the time so we don't know if he actually said that.

In the 6th season episode, Two Fathers/One Son, it is revieled that CSM's real name is C.G.B. Spender. Of course his last name is Spender because since the 5th season we've known that he is Jeffery Spenders father. But what is C.G.B.? I don't know.

CSM was the only member of the sindicate that survived the ambush from the aliens in the 6th season episode Two Fathers/One Son. I don't know if he still has superiors but he did before.

In 7th season episode, En Ami, he reviels that he has an illness and he is going to die. In the season finale, Requiem, he is very sick and has a hole in his throat and in one of the last scenes, Krycek pushes him in his wheelchair down a flight of stairs. We don't know if he is dead but William B. David isn't signed on as a regular cast member for season 8.