positioning on the posted achievements list. Taking that, and the name of the achievement immediately following theirs, "Glory of the Dead", my theory is this.
Perhaps the leadership of the Companions has been infiltrated by Vampires, and "The Circle" is the faction within the Companions that is made up entirely of said vampires.
Thus you make a choice at some point in the Companion's quest line too either join The Circle, I.E become a vampire, or stay human.
This likely pans out into two quest lines with you working with the remaining human members of the Companions too eradicate the Vampires, or, on the reverse side,
working with the Vampires of The Circle too conceal their presence or Vampire-up the rest of the Guild.
What do the rest of you think?