Before the Enslavers
The Outsider has even less history written about it than the Void Dragon, appearing in some legends describing the origins of the wars between the C'tan. It is said that the Laughing God of the Harlequins tricked the Outsider into consuming its own brother C'tan; as it did so, some of those it had consumed remained within, turning it insane. It developed a hellish presence, causing madness in all who came close, and many killed themselves rather than having to face the Outsider.[Needs Citation]
The Outsider shares some similarities with the Nightbringer, in that in it is said that to look upon it would cause terror, much like the Nightbringer's infusion of terror in the young races produced by the Old Ones.[Needs Citation] Furthermore, one of the Harlequins' many dances depicts the moment when the Laughing God tricked the C'tan into consuming each other, except that the C'tan depicted is the Nightbringer, rather than the Outsider.
The Outsider has yet to return to the materium in a recognisable form.