True Organization XIII
The true Organization XIII (真のXIII機関 Shin no Jūsan Kikan ), also known as the thirteen seekers of darkness (13人の闇の探求者 Jūsannin no Yami no Tankyūsha ) is a group consisting of thirteen different incarnations of Master Xehanort that debuts in Kingdom Hearts 3D: Dream Drop Distance. It was first mentioned by Yen Sid in Signs of What's Next when he claims that one Keyblade Master will not be enough to counter what is not "a single one of Xehanort".
The true purpose of the original Organization XIII led by Xemnas was for each of the Nobody members to host a fragment of Master Xehanort's heart, turning them into clones of Xehanort, but the members' unforeseen independence made this goal impossible to realize.
However, in anticipation of this failure, Master Xehanort had arranged for Young Xehanort to be sent by Ansem, Seeker of Darkness to travel to different periods of time and gather different incarnations of Xehanort, who would be added to the ranks of a new Organization. Young Xehanort succeeds in filling twelve of the Organization's seats, and he and the revived Master Xehanort attempt to make Sora into the final vessel. When Sora is rescued by Riku, Mickey, and company, the eleven members aside from Master Xehanort begin to fade away back to their proper times, but Master Xehanort remains confident that in time he will complete his Organization and continue his plans.
There are currently twelve seen members of the true Organization XIII, but only six have been identified so far.