Battlefleet Koronus, officially known as Passage Watch 27-Est, is a small fleet of Imperial Navy vessels charged with the the protection of the Koronus Expanse, a dangerous unexplored region of the Halo Stars located beyond the Calixis Sector in the Segmentum Obscurus, which lies on the northern edge of the known galaxy. At the docks of Port Wander, through the Koronus Passage, and into the Expanse beyond, the grey-hulled and crimson-prowed ships of Battlefleet Koronus defend Imperial interests and provide a bulwark for the Calixis Sector against the horrors that lurk in the depths of the Halo Stars. Officially known as Passage Watch 27-Est, this fleet detachment is nominally under Sector Command, but given a great deal of leeway in pursuing its directives — defend Port Wander and other Imperial outposts and void stations along the Halo Margins, patrol the Koronus Passage, and launch expeditions into the Koronus Expanse to eliminate potential threats to the Calixis Sector. These actions gave rise to the detachment’s unofficial name amongst Navy personnel and civilians alike: Battlefleet Koronus. This squadron, commanded by Fleet-Captain Nathaniel Horne, is on extended detached duty from the rest of Battlefleet Calixis.
From the first discovery of the Koronus Fragments amongst Segmentum Obscuras naval records in 101.M38, the Imperium knew there might be unknown threats beyond the raging Warp Storms at the edge of the Calyx Expanse. These purported records of an unknown Explorator fleet told of mineral-rich worlds, habitable biospheres, and alien empires in the Halo Stars rimward of the Calyx Expanse. As the Angevin Crusade forged those lawless regions into the Calixis Sector, the Imperial Navy probed the edges of the Drusus Marches Sub-sector, searching for passages through the vast storms. None could be found, and the Navy ceased searching in the years after the Crusade, as warships returned to their former battlefleets and the sadly depleted remnants formed the nucleus of the nascent Battlefleet Calixis.
Several centuries later, Sector Command founded Port Wander, a heavily-armed voidstation to serve as a staging ground for Navy operations along the Halo Margins. Navy presence soon dwindled as unrest drew the Battlefleet Calixis’ attentions elsewhere, and the station came under the control of miscreants and Rogue Traders. Even so, when the Rogue Trader Purity Lathimon finally discovered the nigh-mythical Koronus Passage through the Great Warp Storms into the Expanse, Battlefleet Calixis set a watch on this gateway to the Halo Stars. The first Passage Watch Est was led by Terrich Voyle, who commanded the force from the Gryphon, his grandfather’s old flagship.
Affairs continued in this manner until the Ork WAAAGH! Gulgrog smashed through the Maw and laid siege to Port Wander. Though the story of Port Wander’s siege is best told elsewhere, suffice it to say that most Rogue Traders fled or avoided the conflict. The siege broke two years later, when Passage Watch 27-Est rendezvoused with reinforcements from Port Wrath and a fleet sent by the Adeptus Mechanicus from the Lathes. The combined forces defeated the Orks, liberating the imperilled station. WAAAGH! Gulgrog had two lasting effects on Imperial policy. The first was to lend ammunition to arguments that Port Wander should be returned to Imperial Navy control. For decades prior to the Ork attack, prominent individuals in the Imperial Adeptus and Calixis Sector nobility had been petitioning the Lucid Palace on the sector capital world of Scintilla to end the so-called “rule” of Rogue Traders and merchant guilds on this important way point to the newly discovered Koronus Expanse. Port Wander’s near demise (and the haste with which many of its most powerful residents abandoned it in its time of need) was the tipping point in these debates. Within several decades, the voidstation was returned to Navy command.
The second effect was less-noted but would prove equally important. Acting on its own recognizance, Battlefleet Calixis’s upper echelons decided an intermittent patrol was insufficient to defend against incursion through the Koronus Passage. In recognition for its heroism during WAAAGH! Gulgrog, Passage Watch 27-Est was re-tasked as a permanent detachment, operating semi-independently from Battlefleet Calixis. For more than 350 standard years, Passage Watch 27-Est kept this vigil.
Established in 673.M40, Battlefleet Koronus is not large, especially compared to Battlefleet Calixis as a whole. It is comprised of several Cruisers, Light Cruisers, and Battlecruisers, supported by multiple Destroyer and Frigate Escorts. These warships possess most of the combat experience in Battlefleet Calixis, as they are almost constantly in conflict with raiders, pirates, and the occasional xenos vessel that comes through the Maw to plunder the Calixis Sector. Passage Watch 27-Est is hard-pressed but vigilant, and on occasion has sent its distinctive grey-hulled, crimson-prowed vessels on counter-actions and reprisals into the Koronus Expanse.
Today, Battlefleet Koronus operates largely independently of the sector’s overview. It is one of the few fleet detachments not to lose forces to the ongoing demands of the “Margin Crusade,” and has even seen a slight growth in strength as exploration and interest in the Koronus Expanse grows. Defending the sector against the predications of the Expanse is one duty that cannot be allowed to lapse, and since one of Battlefleet Koronus’s tasks is to protect Zone 15 (and the Warp Gate to the Jericho Reach), its ships are so far immune to requisition. Battlefleet Koronus stages from several locations, including Port Wander in the Rubycon II System, Copernicus Depot at the edge of the Drusus Marches, and Fleet Base Metis in Zone 15. However, with the exception of Fleet Base Metis, few warships may be found at these locations at any one time. The majority of Battlefleet Koronus is on constant patrol, the better to project its power.