Thessalon

Thessalon is the mysterious artificial world that acts as the Adeptus Astartes home world of the Keepers of the Accord Space Marine Chapter.

Background
The unassuming exterior of the planetoid Thessalon is the greatest shroud to the great secret that the Keepers of the Accord guard. Though no outsider has ever known the truth, Thessalon is actually an artificial construct of the Dark Age of Technology. It was discovered at some point after the Horus Heresy by an Adeptus Mechanicus Explorator team after it drifted into Imperial space from beyond the galaxy's edge. It has been very difficult to discern who created Thessalon, but what was discovered was that it has been inactive for millennia and that its purpose and function is the absorption of stars and planets as raw material and energy to fashion battle-ready fleets of autonomous vessels. But damning it to tech-heresy is the fact Thessalon's directive is maintained by an artificial intelligence, presumably the same intelligence coordinating the finished fleets the installation produces. However the intelligence appears in hibernation as Thessalon is in active.

Naturally, the Imperium immediately moved to destroy Thessalon for its abhorrent intelligence and heretical nature. The Adeptus Mechanicus took heavy weapons into the construct's winding inner hallways and chasms to obliterate the core systems. Alas, even in its slumbering state, any damage to the installation was almost immediately repaired, and Thessalon's insides shifted under the attack, with panels carrying vital systems being sped away from the intruders. Worse still, no central chamber for Thessalon's power or command center was discovered, so the Adeptus Mechanicus retreated and reported the situation.

The High Lords of Terra responded by assigning an entire Astartes Chapter to protect Thessalon while the Adeptus Mechanicus works to understand and find a way to decommission the installation permanently. This is the source of the Keepers of the Accord's number of techmarines, to help deal with their duty to shield the Imperium against this grave heresy. For if Thessalon were to fully activate it could cause untold damage with its fleets and drain resources that belonged only to Mankind. Thus the current mission of the Keepers of the Accord is to find a solution to disable Thessalon from ever becoming active and to prevent anyone from bring the slumbering leviathan from awakening.

Fortress-Monastery
The Caelestis Castellum, or the "Celestial Castle" in Low Gothic, is the spiritual and martial stronghold of the Keepers of the Accord Chapter. A monastery in the form of a mighty space station, it was gifted to the Chapter by the High Lords to aid in the containment and defense of Thessalon. The Castellum houses enough firepower to eradicate nearly any ship that engages it, to say nothing of the Chapter Fleet surrounding it. Statues and effigies of angelic figures line the Monastery's hull, their shadowed cowls staring eternally outward into the uncaring void. Over time, the Keepers have integrated such iconography and symbolism of the I Legion into the Castellum to honor their forebears, even if it is ignored by the Dark Angels themselves.