- The Grave Warning of The Outsider
The C'tan God known as "The Outsider", known to the other gods as "Tsara'noga", is one of the C'tan (along with the Nightbringer) responsible for consuming many of its brethren during the War in Heaven. In the Outsider's tragic case, he was tricked by Cegorach (instead of the Deceiver) into believing that he would gain the power to rule over the material universe if he cannibalized his brethren (fuck knows why a C’tan would even listen to one of the Eldar deities at all). It is not known whether the Outsider is a dude or a chick. However, of the few fan artworks we have of the Outsider, it seems that almost all of them like to imagine it with dreadlocks, like a psychotic robo-Bob Marley or even as a hot chick, or some kind of immense dragon/leviathan-like creature, rather than taking on a humanoid form. However, C'tan consciousness is apparently so potent that a fragment of each consumed deities' consciousness still remained within the Outsider.
May or may not be a slavic king (TSARa'noga)
The Big Goof
The disjointed thoughts of hunger and half-formed memories of pain constantly claws at Tsara'noga's mind and eventually drove him insane with guilt and rage (it may just be all of the consumed C'tan nagging at the Outsider on how much of a dumbass he is for fucking them over and turning them into Pokemon). In his self-loathing, the Outsider fled the known Galaxy in a self-imposed exile (much like the Silent King, actually). With his eternally tormented mind turned inward, the Outsider's body drifted inert in deep space for millennia, eventually accumulating stellar debris and various exotic minerals, forming a planetoid with the Outsider at its core. A dark planet, floating in the infinitely dark space between galaxies.
But before everybody starts feeling sorry for this... thing, do not forget that all C'tan are immense and extremely powerful entities capable of warping space-time around them, yet the Outsider was strong enough and power-hungry enough to OMNOMNOM all but a handful of them. Its power was so great that it had a hellish presence, and the energy bleed of all the other C'tan it had consumed caused madness in all who came close to it. Many killed themselves rather than face the Outsider. Chaos wishes it could inspire this amount of madness. While the Outsider cannot match the Nightbringer for pure force, or the Deceiver for guile and cunning, it is rumored to be the most deadly and dangerous of the remaining C'tan. This is probably because the rest of the C'Tan gods are divided and Pokeymanned, or dead, and The Outsider is the last intact C'tan, especially after 9e revealed the Void Dragon has also been sharded.
So if the Outsider ever returned from his exile, the Galaxy would promptly shit a metric fuckton of bricks. What else would happen when an extra-galactic immortal ethereal entity of overwhelming power that consumes minds and destroys sanity itself returns for more? Be afraid. Be very afraid.
"Izzit Da Fukkin Buglord?!"
There have been rumors going around the Inquisition that the Outsider is somehow involved with the whole Tyranid affair, and some even claim that The Outsider is actually the Tyranid's Hive Mind. Of course such notion is not only Heresy in the extreme but also quite stupid, as the Outsider has been pretty explicit in not being too interested in Galactic affairs after the incident with him eating his own kin. Plus it would make no sense for a C'tan god to invest in biological engineering. For all the talk of being "the masters of evolution", the Tyranids have been shown recently to not be so infallible as once thought (there are multiple Imperial reports of entire Tyranid Hive Fleets starving to death because they could not find suitable biomass in time). So why the Outsider would decide to create a powerful but flawed biological race when his buddies down the block were creating the sheer rape machines that are Necrons and their subsidiary rape machines (such as their tank one-shoting, armor-melting, atom-destroying, house-cleaning, homework-helping, tax-doing, blow-job-giving guns) is unknown.
Maybe because it's crazy?
All that is known at this time is that more speculation is needed.