Eye of Terror
The Eye of Terror light years-wide black hole of chaotic psychic energy that is also the largest dimensional vortex between the Warp to the Materium and the birthplace of Slaanesh, created from the entire Eldar race's collective unconscious when they participated in a mass psychic resonance of galactic proportions (in layman's terms: They fuck-orgied each other so much all their orgasms merged into a living God Of Rape that ripped open a massive asshole into the very fabric of spacetime itself. Yes it is as hot as it sounds.)
The creation of this galactic laceration killed off the mass majority of the species, with any planets that remain being utterly fucked beyond comprehension after the Birth of Slaanesh. As a result the rest of the Eldar are living on Craftworlds or faraway planets, save for the BDSM addicts who are living inside the Webway. Currently, the Eye of Terror is home to the traitor legions, most of the surviving Primarchs and some unlucky Eldar who still live despite the fall (but more often than not in eternal rape). According to the Warhammer Fantasy 8th Edition Daemons of Chaos army book, the Eye of Terror connects to the same Warp as Fantasy.
It seems in certain fluff, the Eye of Terror was named Cygnus x-1.
What it's like in there
Inside the Eye of Terror, nearly all is subject to the nutty whims of the Chaos Gods and their daemons. What this means is ANYTHING is possible inside this weird vortex. (And not possible, hey it's chaos after all!) So you have worlds in constant fluxes of change; worlds of decay where everything is kept horribly alive somehow; worlds of crystal lit from within by witchfires; and worlds made entirely from the bodies of slaves all merged and molded together... and again, somehow kept alive! It is literally a living physical hell, and any normal person going in without sanity checks will have their brains imploded. There are no maps or regions in this place - just the constant shifting battle-lines as the gods, daemons and their mortal followers battle to claim the worlds for their own.
As mentioned above, a lot of the worlds in the Eye of Terror were the old Eldar homeworlds, now referred to as the Crone worlds. There are lots of goodies still stashed away on these worlds and up for grabs, if you don't mind risking your eternal soul. Despite this, the Eldar (and other individuals cough inquisitors cough) make the risky journey into the eye to acquire some loot, but most end up being corrupted or usually much worse.
Despite it being an endless tempest of raw eldritch power in the very fabric of physical creation, home to some of the most powerful and brutal warriors in the setting, it still does not deter or even frighten the Balls-of-Steel Imperial Guardsmen. This is because of the Cadian Gate.
The Cadian Gate?
Yes, the Cadian Gate.
Y'know how in those war movies, there's that one part where the soldiers are all hanging out partying before the shit hits the fan and they're pressed into service? That's kinda what the Cadian Gate is like to the Eye of Terror. Named after the planet nearest to it, the Cadian Gate is the most stable region around the Eye of Terror owing to the fact that large, strange artifacts of ancient alien design are found all over Cadia and its neighbouring planets. Because of its location in the Eye and because it's the only place not constantly fucked with Warp storms, Cadia (and indeed the entire Cadian Gate) is an enormous flash wherein the Chaos Legions raid the Imperium from their Daemon worlds - only to be completely and utterly WTFPWN3D in an ambush by 100 Baneblades and 75 Imperator Titans hiding behind a lamppost, the biggest of which happened during the Black Crusades.
The Black Crusades
There have been 13 of them in total led by a certain armless failure. Despite his title and the jeering of /tg/ or the retconning of GW, the last Black Crusade managed to really fuck things up - not just for the Imperium, but for EVERY SINGLE FACTION INVOLVED! Chaos Legions splintered and broke off, Guard regiments and Space Marine chapters turned renegade, Warp storms swept the area, Orks are WAAAAGH-ing all over the place (most often against themselves), Tyranids are
going on feeding frenzies reeling from the Chaos fallout and other crazy shit. Serious! Check the numbers from the Eye of Terror campaign and the Disorder forces actually won it. Cadia itself got massively fucked up.
The Disorder forces, however, got creamed in the specialist games, resulting in the Imperial Navy (and the fucking OP Necrons and Eldar) shoving it's metal cock up Abaddon's ass.
Known worlds in the Eye of Terror
- Belial IV: An Eldar crone world, it was one of the capital worlds of the old Eldar empire. Now it is a ghostly shell but rumoured to still house countless Eldar treasures....which of course is the perfect line to have a constant string of the elf-y bastards trying to loot the place without getting their assholes ripped apart in 11 dimensions by Slaanesh.
- Medrengard: The Iron Warriors homeworld in the Eye of Terror. It is basically one big city in constant industrial motion. It is here Perturabo sulks eternally, annoyed his playmate Rogal was silly enough to get himself killed.
- Bubonicus: A daemon world owned by a daemon prince of the same name in service to Nurgle. Visions of this place give psykers across the Imperium terrible
taco-shitsnightmares that probably have some sort of plague mixed in as a bonus.
- Sortiarius, the Planet of the Sorcerers: The world given by Tzeentch to Magnus the Red and the Thousand Sons as their new homeworld after the end of the Horus Heresy. It is a world wrecked by warp storms and with towers thrusting out of the crust of the planet. Magnus mostly stays on the planet in his tower with a big eye on it, which is totally not ripping off Sauron, and plans his legion's campaigns
against the Imperiumfor getting more books to nerd out. UPDATE: No longer in the Eye, because Magnus pulled it into real space. JUST AS PLANNED!
- Plague Planet: The generically named world of Mortarion.
- Oliensis: Devoted to Slaanesh, the entire planet is actually a colossal, morbidly obese man curled into a fetal position. The planet itself will eat you like a Dorito if you try to start shit there, as an unfortunate Space Marine chapter found out during a totally fun company-sponsored vacation trip. Home of the Noise Marines.
- Xana: the first and the biggest of the Dark Mechanicus Hellforges, and the primary place where traitors go to get new ships, tanks, daemon engines, armor, guns and bolts to shoot at loyalist scum and each other. Unlike most other Hellforges that were normal daemon-worlds until DarkMechs colonized them Xana used to be a Forgeworld before the Heresy, and get teleported into the Eye after the Heresy failed in pretty much reverse of the ritual that Magnus used later to teleport Sortarius out.
Important Personages in the Eye of Terror
- Chaos Gods - You can't get higher then the arch-dukes of evil themselves. They each claim the worlds of the Eye as their own and spend every second warring to gain control of them, like supervillains trying to outdo each other in world-conquering plots. Khorne's plots focus on slaughtering everyone off each planet. Slaanesh tries to recreate his super dick move orgasm attack to take over worlds by overloading the inhabitants' very own metaphysical assholes into impossible geometries that would make Euclid's brain asplode. Tzeentch tries to use his clever plots and elaborate deathtraps to take over, even if Failbaddon the Armless is the most frequent victim of such elaborate superdickery. Nurgle just prefers to wait patiently for everything to wear out and rot everything away - good for him, not for anyone else.
- Daemon Primarchs - Each a major or at least legendary player in the Eye, some of the traitor Primarchs have their own worlds while others head the efforts of the different Chaos Gods to claim more for themselves. Because each traitor Primarch is nuttier then a fruitcake by now, they each have different, diverse reasons for warring in the Eye for control.
- Angron - He just wants to kill and rage, as the nails in his daemon-brain make him ever crazier with each passing aeon and his only relief from the rage is MOAR RAGE. Giving worlds to Khorne is just a bonus.
- Fulgrim - No one knows where the fuck he OR his ultimate pleasure world/palace is, but it gives relentless incentive for other traitors to find it. Abaddon met with him on a different planet, so he does get out sometimes, but it's probably pretty rare. After all, with a planet that's supposedly the ultimate pleasure planet, why would anyone want to leave it? Confirmed to have responding to Roboute getting back up by pouting. Fucking weak man.
- Magnus the Red - He rarely leaves his private tower, but he theoretically constantly aids in Tzeentch's vast plans to conquer everything and sometimes comes out to beat up Space Wolves. /tg/ likes to joke that he spends most of the time screaming Just As Planned every time he manages to accomplish common household tasks like dusting the blinds or pouring hellmilk over his daemon-cereal, just to massage his fragile ego after the monumental fuckups his plans seem to keep running into.
- Mortarion - Has his own sweet planet, which he made into a home from home. Like most of the others, he rarely leaves his private quarters, in case his presence makes the plot edge forward. Oh, and the last time he left his planet, a certain someone graffiti'd his heart. Change, after all, is anathema to a Nurgalite. Plus, well... would you invite someone whose B.O. could devastate an entire world without him lifting a finger to your little party? Typhus canonically despises Mortarion for just sitting in his planet and never doing anything.
- Konrad, Horus and Alpharius (maybe) - Too dead to bother with looting and conquering. Jury's still out on Omegon.
- Lorgar - He is meditating. Which he has done for thousands of years. Because Chaos likes to fuck around and give its enemies too many chances. Easily the laziest primarch, and the one with the least excuses for it.
- Perturabo - Mostly stays sulking on his private planet because his sore mood prevents him from getting down to the hard and heavy of fighting. Occasionally he gets off his couch to devastate a Forge World or two.
- Abaddon the Despoiler - Jokes about his incompetence aside, Abaddon didn't get to be the Warmaster of the Black Legion for nothing. If your force runs into his in the Eye, it's probably in your best interest to either submit to the Legion and be subsumed, or run for the hills and pray to your evil gods he's got something better to do.
- Fabius Bile - A key player in the Eye, as he produces clone warriors for the traitor legions to boost their ranks and seeks ways to further improve on their genes. Such biological evil pleases Slaanesh immensely.
During the Gathering Storm the Eye of Terror changed dramaarically. It started with Belisarius Cawl realizing what the Cadian Pylons actually did, so he rushed to Cadia to help. Trazyn the Infinite then rocked up, and helped Cawl try and close it. It started to close, but Failbaddon decided to crash a Blackstone Fortress into Cadia, which destroyed the pylons and allowed the Eye to expand to the point that it enveloped Cadia.