Born the forty-seventh child of Empress Titania Augusta, it was by no means obvious that William would be the one to inherit the throne. The young prince was considered something of an unknown quantity among the Empress’s many children. Famously quiet, bookish, and reserved, it was not until after the Antissa Affair that William’s name was advanced on the long list of candidates for the succession. In the light of history, we may conclude that William’s selection—which was championed by the then-Synarch of the Holy Terran Chantry, Montanus XVII—was made in part because it was believed that the young prince would prove something of a non-entity.

Almost immediately, this perception proved nearly the opposite of correct. Historians have been quick to conclude that the Antissa Affair—which concluded in the murder of Crown Prince Adam—radicalized the young William, who it was said hero-worshipped his much older brother. Within the first year of his rule, William ordered the creation of the Knights Excubitor, an order of genetically perfect homunculus knights designed to be perfectly loyal to William himself. The Excubitors were meant as a check against the Martian Guard’s praetorian tendencies (the Martians having been somewhat implicated in the Antissa Affair and the death of Prince Adam), and helped to shore up William’s own power base in the face of the Chantry, the Exarchs, and the regional governors and lords. Faced with the external threat of the Cielcin, William XXIII poured enormous amounts of capital into the Imperial Legions and Navy, constructing for himself a power base independent of the Cult of Earth and the lesser lords. By dramatically increasing the size and function of the Imperial Office, he constructed several parallel institutions and organizations which were directly answerable to him, and which were staffed by men and women loyal to him, that he might bypass the entrenched authority structures pre-existing his rule. As such, early in his reign, William completely overhauled the Imperial bureaucracy, ousted countless apparatchiks who had persisted in government for centuries, and completely transformed the landscape of life in the Eternal City.

The result was a sort of invisible revolution—so historians of the period all say—one which totally failed to outlive him. Critics of William’s regime, of which many have emerged in the millennia since his death, dismiss him as short-sighted, autocratic, and blinkered. Without William—and indeed without the external threat of the Cielcin—most of William’s novel alterations to the Imperial system collapsed within decades of the Battle of Gododdin, or were forcibly retired by Alexander VI and his regime.

Nevertheless, William XXIII remains the Empire’s most consequential monarch since William I. He is also its longest ruling, reigning from his mother’s death in ISD 15826 to his death at the Battle of Gododdin in ISD 17521, a period of almost 2000 years. William is reported to have spent more than half that interval in cryonic fugue, and indeed spent the latter half of his reign away from the capital—the first Emperor to do so in thousands of years. His visibility and presence at the war front in the latter stages of the Cielcin Wars is largely credited with revitalizing the war effort after the loss of the Veil of Marinus and the Norman provinces, and while his record has become more polarizing in recent centuries, he remains—for many Imperial subjects—the image of the ideal Emperor, and it has become common practice across the galaxy to refer to him as “the Old Emperor” or “the Good Emperor,” and hero cults venerating his memory abound throughout the Empire and indeed beyond.