Ophelia VII
Ophelia VII is the oldest Cardinal World and one of the holiest planets of the Imperium of Man, second only to Terra itself. It is more or less exactly the planet Horus was having nightmares about when Chaos corrupted him on Davin.
Overview and History
Ophelia VII was rediscovered by the Imperium during the Great Crusade and was personally reconquered by the Emperor himself, leading a force of one hundred Custodians and ten thousand Imperial Fists.
After the end of the Horus Heresy and the rise of the Imperial Cult, Ophelia became one of the wealthiest planets in the Imperium, and was the diocese of Benedin IV as a cardinal prior to his assumption of the office of Ecclesiarch in the mid-36th Millennium. When Benedin decided that the High Lords of Terra were cramping the Adeptus Ministorum's style, he decided to pack up and move the entire thing off Terra and onto a new world. Ophelia VII was a natural choice, for several reasons: Benedin was already familiar with the planet from his time as its cardinal, it was wealthy beyond belief, and it was far enough away from Terra that the High Lords wouldn't be able to muck around with the Ministorum. It was during this time that Ophelia came into its own as a Cardinal World. Benedin ordered the construction of unbelievably huge cathedrals on the planet to cement its new status as the center of the Ministorum's power. These cathedrals covered over 90,000 square miles of land and were four kilometers high. For reference, that's three times the size of Portugal and almost enough to cover the entirety of Great Britain. This was only the start, however. Ophelia's distance from Terra allowed Benedin and his successors to raise vast amounts of money through tithing, much of which they poured into turning the planet into the single largest shrine to the Emperor in Imperial space. More enormous cathedrals, bell towers, shrines, and monuments were built, linked by massive avenues lined with statues of Imperial saints. Further, the Ecclesiarchs encouraged cardinals on other worlds to outdo each other in dropping colossal sums of money on their own mega-cathedrals, statues, and shrines. Deviant cults and heretics were rooted out and exterminated with extreme prejudice, and the Ministorum slowly consolidated its power over all matters spiritual in the Imperium. After three hundred years, Ecclesiarch Greigor XI decided to move the whole production back to Terra for no apparent reason. It took twelve years and a decent chunk of the money in the Ministorum's treasuries, but the Ecclesiarchal Palace was once again made ready to host the spiritual leaders of the Imperium.
That One Heretical Fucker and the Sisters of Battle
During the Age of Apostasy, Goge Vandire's tyrannical reign over the Imperium caused some of his opponents in the Ministorum to try and flee for Ophelia VII; for reasons unknown, their ship was lost somewhere in the Warp. Once Vandire had been thoroughly BLAMMED by Alicia Dominica and Sebastian Thor was kicked into the big chair to replace him, he decided to set up the Synod Ministra on Ophelia. The purpose of this new organization was twofold. First, it would allow the Ministorum's decrees to be propagated more easily across Imperial space. Second, by serving as a second center of power for the Ministorum, it would ensure that no single individual, not even an Ecclesiarch, could ever seize control of the entire organization. Apparently Thor had been paying attention to Guilliman's ideas. This meant that the newly formed Adepta Sororitas would also be split between the two worlds, again to ensure that no one individual could control the Ecclesiarchy's entire army of hymn-singing bolter bitches. Therefore, the Convent Sanctorum was founded and headquartered on Ophelia VII. This makes it the home base of three of the six major Orders Militant: the Order of the Bloody Rose, the Order of Our Martyred Lady, and the Order of the Valorous Heart, along with half of the Orders Famulous, Dialogous, and Hospitaller. Two famous members of the Sororitas also hail from Ophelia VII, Saint Praxedes and Saint Helena the Virtuous.
41st-42nd Millennium
Ophelia was hard-hit by the forces of Chaos during the 13th Black Crusade and the eruption of the Cicatrix Maledictum. The planet and its system were invaded and actually conquered by a Lord of Change known as the Tyrant of Blueflame. The Sisters of the Order of Our Martyred Lady fought furiously against this desecration, but the Tyrant's forces were overwhelming and they sustained nearly 50% casualties. The Sisters were on the verge of resigning themselves to a glorious death when Grandpa Smurf's Galactic Comeback Tour and the Black Templars came charging in at the last moment, smashing the Tyrant's forces and driving them off the planet. Guilliman left a garrison force of Black Templars under Marshal Armond Montfort in the system to guard against further incursions. Unfortunately, they were called away to help Saint Celestine reconquer the planet of San Leor, and the Tyrant promptly attacked the system again, this time invading the planet of Ophelia IV with the aid of the Alpha Legion and a shitload of cultists. His goal was to use the planet to complete a Warp ritual that would allow him to unleash a horde of daemons on Ophelia VII, but Celestine herself turned up and kicked his feathery ass out of the system.
Trivia
In the early days of 40K canon, Ophelia VII was instead known as Ophelia IV or 4, though nobody seems to know why it got changed. Good chance it was a typo that no one caught until it was too late.
The whole thing with the Ecclesiarchy packing up and moving to Ophelia VII for a few centuries is most likely based on the Avignon Papacy of 1309-1376, when the French king Philip IV strong-armed the cardinals of the Catholic church into electing his close friend Bertrand de Got as the new Pope, who then refused to move to the Vatican and set up shop in the city of Avignon. The reasons for this are complicated and deal with the role of the state vs. the church in temporal/political matters, but it basically boils down to Philip wanting to prove a point about how nobody told him what to do when it came to running his kingdom, sort of the reverse of the 40K scenario.