Eye of Terror
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A black hole-like space vortex of chaotic psychic energy that is also the largest gateway from 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 retarded as it sounds.) The area of the Eye of Terror used to be the area where the old Eldar planets used to be but now they're fucked beyond comprehension after the Birth of Slaanesh so the rest of the Eldar are living on Craftworlds or faraway planets....well except for the BDSM addicted spehss elves who are living inside the webway. Currently home to the traitor legions, most of the surviving Primarchs and some unlucky Eldar who still live despite the fall.
So What's it 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 (and not possible, hey it's chaos after all!) inside this weird vortex. So you have worlds in constant fluxes of changs; worlds of decay where everything is kept horrible 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 kept somehow alive!
It literally is a living physical hell and any normal person would go insane quite quickly from being in there. There is no maps or regions in this place; just the constant shifting battlelines 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 is 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 inquistors cough) make the risky journey into the eye to acquire some loot but most end up being nommed or usually much worse.
Whenever things get too hot for the traitor legions and their goons after some nice pillaging, they always retreat back to the Eye of Terror and mock the Imperium saying "Nah nah, you can't get us," to which the Imperium always replies "If we could then we would've slaughtered all of you heretics long ago," and the traitors have of mixed response of "No you wouldn't" and "Yes you would've, but you can't, so haha."
Despite it being a 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 (see below)
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 pwned by a Baneblade or other similar OP superweapon sneaked right behind them in an outhouse, the biggest of which happen during the Black Crusades.
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), Tyranid feeding frenzies 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.