Guardian (or Defender, for members of the light side path), is the sixth rank in the Dark Brotherhood, which is awarded after completing the requirements of a Protector.
It is part of the Journeyman class, the sixth step down the path to Dark Jedi Knight.
The Requirements to be promoted past this rank are as follows:
The requirements for the promotion from Guardian are found at the above link, the completion of the above requirements will earn a promotion to Jedi Hunter.