13th Black Crusade
In 999.M41, when Abaddon the Despoiler finally launched his 13th Black Crusade, the greatest Chaos assault on the Imperium since the Horus Heresy, the Forces of Chaos managed to make landfall upon Cadia itself and occupy large swathes of the planet despite ferocious Imperial resistance. Led by the Lord Castellan Ursarkar E. Creed, the Imperial forces were ultimately able to contain the Chaos assault but they could not force the Traitor Legions to leave the planet. In the space above Cadia, the Imperial Navy proved victorious over the Chaos warfleet led by Abaddon while the Eldar provided a timely assist in facing off against a Blackstone Fortress inhabited by a sentient avatar of Slaanesh himself. At the end of that campaign, the Imperial Navy controlled the space over Cadia and thus prevented the Forces of Chaos from breaking out of the Cadian Gate, yet at the same time the Traitor Legions and their allies remain in control of a large amount of Cadian territory. They have dug in and erected elaborate defences that will prove very difficult for the Imperial forces to overcome. As a result, at the present time, a bloody stalemate reigns on the Imperium's most important Fortess World, though if the Forces of Chaos were ever able to seize the orbital space over Cadia, they might at last gain the advantage necessary to fully conquer the planet and overwhelm the remaining defences of the Cadian Gate. Should they do so, then the Imperium may face the greatest threat to its existence since the Horus Heresy, 10,000 years ago...
Post-Heresy
Following the Battle of Terra that ended the Horus Heresy with Horus' death and the internment of the Emperor of Mankind in the Golden Throne, the defeated Traitor Legions and their allied forces among the Imperial Army and the Dark Mechanicus fled from Terra. Some of the exhausted Loyalists rallied and gave chase, but most remained on Terra to consolidate their great victory over the Forces of Chaos. Many of the surviving Traitors were put to the sword, but the majority of the Traitor Legions escaped into the great Warp rift known as the Eye of Terror in the Segmentum Obscurus, a region of space where reality and the insanity of Chaos collide as the raw psychic energy of the Immaterium pours into real space-time.
Within the Eye of Terror, the Chaos Gods rule over uncounted numbers of planets, all warped to reflect their own dark aspects. It was there that the Traitor Legions found refuge, isolated from the rest of the galaxy by potent Warp Storms. Each of the planets within the Eye is a Daemon World, warped and twisted by the whims of the Ruinous Powers and the powerful Daemon Princes who rule over them in the Dark Gods' name. The Chaos Space Marines regrouped and nurtured their hatred of the Imperium, planning for the day when they would wreak a terrible vengeance on those who had defied them and their foul masters. Within the Eye time flows differently than in realspace. Those same Traitors who fought on Terra 10,000 standard years ago still fight today in the service of Chaos. They fight against each other to prove their supremacy and against the forces of the Imperium when the Warp Storms calm enough to allow them to emerge into Imperial space.
The Imperial sectors surrounding the Eye of Terror are heavily militarised to resist these frequent invasions and none more so than Cadia, the Imperial Fortress World that stands at the very mouth of the only stable navigational route leading out of the Eye of Terror, the dreaded Cadian Gate.
As a result of Abaddon the Despoiler's 1st Black Crusade in 781.M31, the planet's strategic location was deemed vital to the defence of the Imperium, and so, in the 32nd Millennium Imperial colonists were dispatched to resettle the world, becoming the ancestors of the present-day population of Cadians. Perhaps as a result of the Eye of Terror's proximity, this later population of Cadians also soon developed the unusual violet-coloured eyes that had marked the first human inhabitants of the planet.
The early defences of the newly resettled Cadia proved to be woefully inadequate. Its major cities were extremely vulnerable to enemy assault as they had been constructed in the traditional High Terran style, with broad, ordered avenues. Following the 2nd Black Crusade in 597.M32, sweeping changes were carried out worldwide to improve the planet's overall defensive capabilities, and massive fortifications were constructed across the world until the planet's cities had been rebuilt into their current form.
Cadia stands upon the only known reliable route out of the Eye of Terror and thus is one of the most strategically vital worlds in the entire Imperium of Man. There are other routes out of the Eye, but none are stable like the Cadian Gate and no military force of any true size can venture forth from the Eye without first passing through it. The exact reasons for the existence of this unusual region of stability is unknown, though many Magi of the Adeptus Mechanicus believe it is due to the presence of the famous Cadian Pylons. These mysterious black monoliths, now known to have been created by the Necrons millions of standard years ago to hold back the psychic influence of the Warp that was so feared by their C'tan masters, dot the landscape of Cadia and their origins remained mysterious until the time of the 13th Black Crusade.
Cadia itself is a bleak, merciless and wind-blown planet, where only the strongest survive to adulthood and discipline is learned from the moment a babe takes his or her first steps. Cold winds howl across wide, sundered plains where armies train with live ammunition and every day not spent training is believed to be a day wasted. Every Cadian fortress-city, or "Kasr", is a great citadel, with the streets and buildings fashioned with great tactical cunning by the finest military engineers and siege specialists of the Imperial Guard. Every Cadian is taught the skills of the warrior as soon as they can walk and they are much sought after by commanders throughout the galaxy. Cadian military gear is considered top-rate and is used as the standard for all Imperial Guard Regiments. Such a world breeds hardy and determined warriors and the Cadian Regiments of the Imperial Guard have a well-deserved reputation for both honour and fighting spirit. From the earliest age, Cadians are taught to field-strip a weapon with their eyes shut and tactical doctrine is taught before basic literacy. One soldier in every ten is recruited into the Cadian Interior Guard, regardless of ability or achievements, and as a result some of the most able soldiers spend their entire Imperial military service on Cadia and the soldiers of the Cadian Planetary Defence Force are amongst the most skilled fighting men in the Imperium, the equal of many other worlds' Imperial Guard Regiments.
Pre-Heresy
Some 40 standard years before the outbreak of the Horus Heresy, Cadia was a world inhabited by a primitive race of violet-eyed humans who worshipped the four Chaos Gods, probably a remnant of Mankind that had turned to the Ruinous Powers during the hardships of the Age of Strife. Prompted by the so-called Pilgrimage of the Primarch Lorgar of the Word Bearers Legion to discover whether or not the Gods once worshipped by adherents of the Old Faith of the Word Bearers' homeworld of Colchis actually existed, Lorgar journeyed with his Word Bearers Legion's Chapter of the Serrated Sun to what was then the fringes of known Imperial space as part of the 1301st Expeditionary Fleet of the Great Crusade. At this time, Lorgar had not yet fallen to Chaos, though he had turned against the Emperor of Mankind as a deity no longer worthy of his worship after the Emperor and the Ultramarines had personally humiliated him and the entire Word Bearers Legion on the world of Khur 43 standard years before the start of the Horus Heresy. The Emperor had come to Khur personally with Malcador the Sigillite after ordering the Ultramarines to destroy the Khurian city of Monarchia where the Emperor was worshipped as a God as a result of the teachings of the Word Bearers. He made his displeasure known to Lorgar about the Word Bearers spreading the religion of Emperor-worship to every world they brought into the Imperium, in direct contravention of the rationalist, atheist philosophy of the Imperial Truth. The Emperor forced the entire Legion to kneel against their will through the use of his psychic might and then explained that they were the only Astartes Legion to have failed his purpose on the Great Crusade. After this humiliation Lorgar, on the advice of his First Captain Kor Phaeron and the Word Bearers First Chaplain Erebus, decided to undertake a Pilgrimage to discover if the Gods worshipped by the ancient Old Faith of Colchis were real and worthy of the Word Bearers' faith and allegiance.
The Word Bearers were also accompanied on this Pilgrimage by 5 members of the Adeptus Custodes who had been set by the Emperor to watch over everything the Word Bearers did to prevent them from falling back into error once more. The 1301st Expeditionary Fleet exited the Warp near the largest Warp Storm in the universe, later known as the Eye of Terror. The Fleet's Master of Astropaths advised Lorgar that unusual "voices" in the Warp were heard in the vicinity of the great Warp rift, voices that spoke directly to the Primarch as well, the voices of the Chaos entities within the Immaterium.
The decision was made to hold orbit over Cadia and for the 1301st Fleet's elements to make planetfall on the unknown world, designated as 1301-12. The landing force was comprised of Imperial Army, Word Bearers, Adeptus Custodes and Legiones Cybernetica elements. The landing party, led by Lorgar, was greeted by a large number of barbaric human tribes, tribes described as "dressed in rags and wielding spears tipped by flint blades...yet they showed little fear." Most notable were the barbarians' purple eyes, which reflected the colour of the Eye of Terror itself in the spectrum of visible light. Despite the Custodian Vendatha's protests and request to execute the heathens, the Word Bearers approached the natives. A woman emerged from the crowd and addressed the Primarch directly, calling him Lorgar Aurelian and welcoming him to Cadia. This woman, the priestess Ingethel, would ultimately lead the Primarch down a path of spiritual enlightenment that actually marked the beginning of Lorgar's fall to heresy and Chaos. Later, Ingethel of Cadia would lead the 1301st Fleet's scout vessel Orfeo's Lament into the Eye of Terror and thus change the Word Bearers forever as they were exposed to the Ruinous Powers of Chaos and slowly corrupted, the first of the Legiones Astartes to worship the Chaos Gods and become Traitors to the Emperor. The Cadians, primitive as they were, used a language which was akin to the Word Bearers' own Colchisan tongue. Many traditions of the Word Bearers were mirrored by the culture of ancient Cadia, leading Lorgar to believe that the original settlers of both his own homeworld of Colchis and Cadia shared a common heritage.
Following the visits into the Eye of Terror, Lorgar ordered a cyclonic bombardment of the planet, wiping out the Cadians and leaving the planet abandoned, so none within the Imperium would know what had transpired there.