The Outsider, whose true name is Tsara'noga, is one of the four remaining C'tan, the Star Gods of the Necrons. The Outsider is probably the most mysterious of the four and very little is known about it. The Outsider, like the other remaining C'tan, was tricked by Cegorach, the Eldar Harlequin's Laughing God into feasting upon its fellow C'tan. However, for reasons that can only be speculated at, a fragment of each consumed deity's consciousness remained and plagued the mind of the Outsider, driving it insane. In its insanity it left the populated area of space and now wanders out in the void, where it fights its inner demons and feeds once more upon the stars that gave birth to it. It also had a hellish presence, and it caused madness in all who came close and many killed themselves rather than having to face the Outsider. While the Outsider cannot match the Nightbringer for pure force, or the Deceiver for guile and cunning, it is rumoured to be the most deadly and dangerous of the four remaining C'tan. Should the Outsider return from its self-imposed exile, the galaxy shall bear witness to a culling not seen since the last Necron harvest, which almost wiped out all life in the galaxy and brought about the downfall of the Old Ones.
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. Also, another source gives a dance by the Harlequins of this precise moment when the Laughing God tricked the C'tan into consuming each other, except the C'tan in the story of the dance who was tricked is most definitely the Nightbringer. These similarities are not backed up specifically by any background information and it is therefore unlikely that they are one and the same as there are sources that show many differences between the two entities.
Since the the recent invasion of the galaxy by the Hive Fleets of the Tyranids, it has been speculated by some in the Inquisition that the Outsider is behind the Tyranids' arrival in the Milky Way. He may be the driving force behind the unified Tyranid Hive Mind, as he seeks to destroy the galaxy's defenders with the living weapon that is the Tyranid species. If he is the Tyranids' ultimate master, then there may be no stopping the Tyranids from consuming the Imperium of Man and all who would oppose the C'tan. The Necrons will fully awaken and shall once more harvest all life in the galaxy.
- Codex: Necrons (3rd Edition), pp. 5, 25, 63