Treya was little more than a footnote in the annals of the Ullanor Crusade. Neither Ork nor Astartes set foot upon its soil, nor was its system marked by any great battle or tactical significance. Of its five planets—only one held any promise for habitation. Even then, only half of the planet’s surface was considered viable for colonization, the rest locked in an endless cycle of freezing and flooding due to the planet’s extreme axial tilt. Where ice did not entomb the world, tidal deluge followed, reshaping its surface with a rhythm as merciless as it was predictable.

During the Great Crusade, a small complement of Ultramarines deposited a Mechanicus survey team to begin limited terraforming on the single continental landmass, naming the planet Treya. But as the war moved elsewhere, Treya was forgotten. Left to develop at its own pace, the world endured.

It would be many centuries later—long after the fires of the Horus Heresy had cooled and the Imperium had solidified into its modern form—when Treya once again received the gaze of the Emperor’s Angels of Death. This time, it was not the Ultramarines, but the sons of Rogal Dorn, the Imperial Fists, who descended upon the ice-locked world.

By then, Treya’s initial terraforming efforts had borne fruit. A single hive city, Hydromos, had been raised atop the planet’s sole stable continent, serving as both heart and spine to the world’s economy. In a cruel twist of planetary misfortune, Treya’s oceans would freeze solid for half the cycle, only to melt into swelling floodwaters during the other. Yet in that brutal cycle, the people of Treya found purpose: ice was easier to quarry, store, and ship than water.

Hydromos and its surrounding agri-colonies grew rich exporting massive quantities of cryogenically compressed ice to nearby systems starved for reliable water supplies. But that prosperity came at a cost—the labor was treacherous, the deaths frequent, and the wealth unequally shared.

When the Imperial Fists walked the ice platforms, domed colonies, and flooded outer sanctuaries of Treya, they saw something rare: a people hardened not by war, but by endurance. The culture was one of tenacity, stoicism, and defiance in the face of nature itself. The Chapter’s Reclusiarch reportedly described the Treyan people as “already knowing the weight of sacrifice before being asked to carry it.” In Hydromos, they saw potential—and a world worth guarding. Treya was quietly marked for future consideration.

It was during the 9th Founding that the decision was made.

The Imperium returned to Treya in force, bringing with it the full weight of the Mechanicus, the Adeptus Administratum, and the sacred gene-seed of Dorn. New forges were constructed at the planet’s northern pole, one of the only regions to remain perennially frozen even amidst Treya’s wild seasonal swings. Beneath these frozen wastes, a foundation was laid for the chapter’s future—the Fortress Monastery of the First Wall.

Despite the lack of solid ground beneath the ice shelf, the Mechanicus succeeded in anchoring their macro-structures deep into the frozen strata. Massive reactors, training halls, armories, and war-foundries were constructed—each a cathedral to war. The forges of Hydromos were likewise retooled to support the chapter’s broader needs and to boost interstellar trade, the Mechanicus claiming their tithe in wealth and influence in return.

With the seal of gene-seed approval granted, the sons of Treya ascended.

From the cold was born a new brotherhood of warriors—the First Wall.

Where once Treya had been a barren, overlooked world, it now stood as a bastion of Imperial might, its sons clad in ceramite, its industry bent toward war. Treya had been patient, enduring the cold, the dark, the centuries of silence.

It had waited long enough.