Saint Katherine is the patron saint of the Order of Our Martyred Lady of the Adepta Sororitas, which was originally known as the Order of the Fiery Heart. She was one of five companions of Alicia Dominica, the patron saint and founder of the Adepta Sororitas. Known as the "Shield Bearer," Katherine was regarded as Alicia Dominica's second-in-command and was the bearer of Dominica's Praesidium Protectiva -- a large shield said to contain a shard of the Emperor's own personal Power Armour, that was impervious to physical attacks. Katherine was well-known for her fiery determination and vengeful spirit, ever leading her Sisters at the forefront of innumerable battles and pursuing the prosecution of numerous Wars of Faith against Heretics in the name of the Emperor of Mankind. She was the founder of the Order of the Fiery Heart. Her Sisters came to reflect her vengeful and fiery nature. Leading her Order for many decades, she was declared a Living Saint within her lifetime for performing legendary acts. Eventually, Katherine met her end at the hands of evil men when she was murdered by the Witch-Cult of Mnestteus. So deeply did her Sisters mourn Katherine's loss that they renamed their Order the Order of Our Martyred Lady. Unknown to her Order, sometime before her untimely death, Saint Katherine visited the world of Sanctus Lys in the Palatine Sector. Beneath a shrine, in a hidden crypt, she interred her golden Power Armour and her legendary Power Sword, the Ardent Blade. These powerful artefacts were found centuries later and retrieved by the Living Saint, Saint Celestine.
Katherine's origins can be traced back to the late 35th Millennium, during the Age of Apostasy, and the rise of the Apostate High Lord Goge Vandire. Through political maneuvering, blackmail, and outright murder, Vandire managed to assume the mantle of both High Lord of the Administratum and Ecclesiarch of the Adeptus Ministorum. Vandire sought to impose his own rule over the Imperium in place of the Emperor and used the power of the two Adepta he controlled to eliminate anyone who opposed him. Thus began the period in Imperial history known as the Reign of Blood. Through trickery and manipulation, the newly discovered sect known as the Daughters of the Emperor from the world of San Leor were co-opted into becoming Vandire's personal bodyguard, and were renamed the Brides of the Emperor. When the Holy Synod of the Ecclesiarchy attempted to have Vandire assassinated a few years later to rid themselves of the tyrant, the Brides went into the Synod's meeting chambers, locked the doors, and emerged an hour later carrying the severed heads of every Cardinal present. This violent repression and wanton slaughter continued for seven decades after Vandire's ascension.
Soon rebellion against Vandire's tyranny began to foment after a simple preacher named Sebastian Thor, from the world of Dimmamar, began to rail against the excesses of Vandire and called for a holy Crusade to restore the Emperor's rightful rule. At the height of the Age of Apostasy in 378.M36, an elite Space Marine strike force composed of the Imperial Fists, Black Templars, Fire Hawks, Soul Drinkers, and the forces of the Adeptus Mechanicus assaulted the renegade High Lord Goge Vandire's seat of power -- the Ecclesiarchal Palace on Terra. Ultimately, it was not the armies of the Space Marines and the Tech-priests that brought about the doom of High Lord Vandire. It was his most trusted companions, the sisterhood of the Brides of the Emperor. Only one Imperial Adepta had remained apart from the bloodshed and devastation of the era. Within the secure walls of the Imperial Palace, the Adeptus Custodes continued their eternal vigil over the Golden Throne. To escape the anarchy that prevailed in the wider Imperium, and to ensure the protection of the Emperor Himself, the Custodians had cut themselves off from the outside completely. Only scraps of information passed through the sealed walls of the most sacred of places in the galaxy, and it was only when the Space Marines and the Adeptus Mechanicus moved against Vandire that the full extent of the treachery perpetrated by the High Lord became known to them. In secret meetings with the commanders of the Space Marines, the Adeptus Custodes learned of the Reign of Blood and how the Brides of the Emperor defended the traitorous High Lord. The mysterious order advised the Space Marines to continue their attack while they would do what they could.
The Captain-General of the Custodes led a small force through the secret passages deep beneath the surface of Terra. After many days of travel, the group emerged in the bowels of the Ecclesiarchal Palace, and approached Alicia Dominica, the leader of the Brides of the Emperor. Calling for a truce and a parley, the Centurion laid down his weapons and walked unarmed to meet the guardians of Vandire. For an hour he made an impassioned plea for the Brides to revoke their oaths, striving to convince them that they were fighting for evil, not the Emperor. However, they were not to be swayed by his arguments. The nameless Centurion had only one option left. Leaving his men as hostages, the Centurion guided Alicia Dominica and her personal bodyguard of five female warriors (Arabella, Katherine, Lucia, Mina, and Silvana), into the heart of the Imperial Palace, the Sanctum Imperialis, to stand before the God-Emperor upon his Golden Throne.
What occurred in this most sacred of chambers is not recorded, but when the Brides of the Emperor stepped through the Eternity Gate once more into the Outer Palace, their eyes burned with unparalleled anger and hatred. Without a word, the Centurion led them back through the dark places of the earth, this time leading them directly to Vandire's Audience Chamber. Alicia Dominica spoke of the treachery of Vandire and his depraved corruption of the Ecclesiarchy, but most of all she spoke of his twisted perversion of their own Order. Burning with shame and anger, they renounced the name of the Brides of the Emperor and once again became the Daughters of the Emperor. Alicia Dominica and her vengeful Sisters confronted the corrupt Vandire within his own chambers. Dominica pronounced judgment on the apostate Vandire, and in front of 4,000 assembled Brides of the Emperor, drew her Power Sword and held it aloft for all to see. Vandire glanced around the assembled warriors, his brow knitted in confusion. In the end the High Lord appeared so divorced from reality that he could scarcely comprehend Alicia's words. The Power Sword slashed down, beheading the traitorous High Lord in one stroke. The Reign of Blood had ended.
Despite the end of the Reign of Blood, the Age of Apostasy was to rage for several more centuries after the death of High Lord Vandire as a result of the political destabilisation that he had caused. With the appointment of Sebastian Thor as the new Ecclesiarch, he unleashed a reformation of the Ecclesiarchy and ended the wars of the religious schism, giving birth to the Inquisition's Ordo Hereticus, intended to hunt for the enemies of the Emperor within the Imperium. They owed their power in no small part to Thor’s famed Decree Passive that forbade the Ministorum from raising permanent bodies of “Men under arms." Following the disbanding of the Ecclesiarchy’s standing armies and fleets (the Frateris Templar), the Daughters of the Emperor, now called the Adepta Sororitas, replaced the Templar as the defenders of the Ecclesiarchy and the faith of the Imperial Cult from direct threats. Alicia Dominica formally established the Sisterhood -- the Orders Militant of the Adepta Sororitas, with the Orders of the Sisters divided between the Convent Prioris on Terra and the Convent Sanctorum on the Cardinal World of Ophelia VII.
Dominica then became the first Canoness of her own Order Militant, the Order of the Ebon Chalice.
Katherine would go on to found her own Order Militant, the Order of the Fiery Heart. Her Sisters came to reflect her vengeful and fiery nature. Leading her Order for many decades, she was declared a Living Saint within her lifetime for performing acts that would become legendary even amongst an entire galaxy of legends. But such legends seldom end happily, and first Dominica, then Silvana and Lucia were martyred at the hands of evil men, men jealous of their faith and purity. Katherine was murdered by the Witch-Cult of Mnestteus. So deeply did her Sisters mourn Katherine's loss that they renamed their Order Militant the Order of Our Martyred Lady. Unknown to the Order, sometime before Katherine's untimely death, she had visited the world of Sanctus Lys in the Palatine Sector. Beneath a shrine, in a hidden crypt, Katherine had interred her golden Power Armour and her legendary Power Sword, the Ardent Blade. These powerful artefacts were eventually found centuries later, retrieved by the Living Saint, Saint Celestine, during the Palatine Crusade.
- Codex: Sisters of Battle (2nd Edition), pp.18-19, 35
- Codex: Witch Hunters (3rd Edition), pp.6-7, 19, 47
- Dark Heresy: Blood of Martyrs, pp. 10-13, 41