The Ulgroth Crusade was a conflict fought in the Ulgroth Sector, an Imperial-controlled region neighboring the Cadian Gate. The conflict began with a rebellion on the oceanic world of Oseonus III in 960.M41, which quickly escalated into a full-blown invasion of the planet by the forces of Chaos. After successfully claiming control of Oseonus III, Chaos forces then commenced an invasion of the Ulgroth Sector as a whole. A number of worlds fell to Chaos before the invading forces were eventually fought to a stalemate in 975.M41 thanks to the intervention of over twenty Space MarineChapters, as well as the valiant efforts and strategic skill of Admiral Quentin Holgrim of Battlefleet Ulgroth.
The Ulgroth Crusade broke out during the turbulent times preceding the beginning of the 13th Black Crusade. Though morale amongst Imperial forces in the Segmentum Obscurus was at a high point thanks to the recent Imperial victory in the bloody Verdictus Crusade in the previous year, unrest in the region was also mounting. Chaos activity around the Eye of Terror was well above the norm and increasing rapidly, and space hulk sightings in and around the Cadian Sector were at an all-time high. Cults of apostasy erupted through the regions near the Eye as many began to claim that the Emperor had forsaken them. This in turn allowed for the seeds of heresy to take hold on many planets in the area.
Unrest in the Ulgroth Sector came to a head when Governor Westmund Trant of Oseonus III declared his world's secession from the Imperium of Man. Oseonus III, an entirely oceanic world, tithed fresh drinking water that was critical for numerous nearby systems as well as to the Imperial Navy. With important resources at stake, the Imperium was quick to react, dispatching the Black Lions Space Marine Chapter as well as several Imperial Guardregiments and an Adepta Sororitas Commandery to Oseonus III. However, upon arrival the situation was discovered to be far more grave than anyone thought: rebel forces were opening Warp rifts in attempts to summon daemons as well as to allow fearsome Traitor Astartes to come and aid them in their fight. With the risk of a possible daemonic incursion imminent, the Black Lions called upon the War Claws and Aetherian Warriors Chapters for aid.
When the War Claws and Aetherian Warriors made landfall on one of Oseonus III's buoyant float-cities, specifically Hydroth, they were met with extremely heavy resistance from rebel forces, with a minor Daemon Prince even being summoned, although it was unable to wreak much havoc before being beheaded by CaptainCorradino Breda of the Aetherian Warriors. Disturbed by the level of resistance encountered during their initial assault, Captain Breda called for additional reinforcements from the Doom Eagles, who obliged Breda with a strike force from their Fourth Company, headed by Breda's long-time acquaintance, Veteran SergeantEgon Talorus. In the meantime, the Black Lions and a small detachment of Aetherian Warriors would focus their efforts on cutting off the rebels' access to supply depots on the float-city of Osith Minor, while Breda and Sergeant Sabato Lepore of the War Claws would focus on making plans for taking the capital, Osith Prime, from which the rebels were using artillery batteries to bombard loyalist forces now massing in Hydroth. And once the Doom Eagles strike force arrived, their plans were put into action.
However, as the assault on Osith Prime was beginning to get underway, the Aetherian Warriors contingent from Osith Minor, headed by Veteran Sergeant Halius Aetherius, unexpectedly arrived, informing Breda that they had lost Osith Minor, only narrowly escaping with their lives as an massive unknown being emerged from the sea. Just then, a Chaos warfleet, headed by KhornateChaos LordAahrgoth the Conqueror, emerged over Oseonus III. Aahrgoth immediately dropped troops onto Osith Prime, heading them himself, and declared his intention to sink the city and kill everyone in it in one massive sacrifice to the Blood God. Meanwhile, in orbit, Aahrgoth's fleet began tearing into the now grossly outnumbered loyalist naval forces.
Breda now began to engage Aahrgoth in melee combat, while Egon Talorus' Doom Eagles and Halius' remaining men hurried to defend the city's buoyancy regulators. Lepore, however, frustrated by the turn of events, made a reckless attempt to aid Breda, spoiling a potential killing blow to Aahrgoth from Breda's power axe, before being struck in chest by the Chaos Lord's weapon, forcing Breda to abandon the fight in order to bring the wounded Lepore to safety. Meanwhile, Egon's forces were managing to hold off Chaos forces from damaging one of the regulators, but Aetherius' men, already battered by the events in Osith Minor, were overrun, allowing the other regulator to be disabled, causing the city to begin slowly tipping sideways into the sea. But as the War Claws dispatched their Techmarines to attempt to repair the regulator and aid Aetherius, an enormous Bloodthirster of the Sixth Host emerged from the sea and began to attack Egon's forces. The Doom Eagles were quickly overpowered and forced to abandon their regulator, which the massive daemon promptly smashed.
The surviving Astartes quickly made an escape from Osith Prime as it was swallowed by the sea, returning to their ships in orbit as daemons now began to swarm Hydroth as well. Hopelessly outnumbered, the Astartes vessels hastily abandoned the system as deadly Warp storms ripped open around Oseonus III.
After the Chaos victory at Oseonus III, renegade incursions began to break out across the sector, and more warfleets arrived in the northwest from the Eye of Terror. Fearing that the start of a Black Crusade was imminent, a number of Space Marine Chapters dispatched forces to the Ulgroth Sector, while the Departmento Munitorum deployed large numbers of Astra Militarum forces.
A sacred desert Shrine World reliant upon Oseonus for water supplies, Lenatix IV was impacted heavily by the fall of Oseonus III. The planet quickly began to fall into anarchy, with the population fighting itself over what little water remained. Ultimately, control of the planet's water fell into the hands of a secret cult of unsanctioned psykers, who used their unholy witchcraft to put down any opposition. Impressed by the skill of these seemingly self-trained psykers, TzeentchianSorcererLord Mordenath the Knowing offered water and protection to the people of Lenatix IV in exchange for the psyker cult's cooperation.
The Aetherian Warriors, Jade Falcons, and Sons of Eleusis Chapters each deployed strike teams to Lenatix IV in hopes of breaking the psyker cult and returning the world to the Emperor's rule. Unfortunately, Mordenath had backed the cult with the full strength of his warband of mighty psionic warriors, the Whisperers of Xeriphos. The Whisperers kept the loyalists held back under the weight of fire from their wicked Warp-fueled weaponry and psychic attacks, as their master began to conduct a sinister ritual to consume the world in a blaze of sorcerous Warp flame.
Eventually, the loyalists would manage to push on through the bulk of the Whisperers' defenses, only to find that Mordenath had summoned a mighty daemonic horde to his aid, led by a fearsome Lord of Change. The Feathered Lord kept the Marines at bay with a legion of Horrors, Flamers and Screamers, all the while blasting away at them with bolts of blue and pink lightning. But the loyalists were resolute, and managed to push on through the daemons to face Mordenath himself.
But they were too late. Mordenath finished his twisted ritual just as the Marines came to face him, a magical inferno erupting to consume the planet as Mordenath cackled in celebration of his victory. The loyalists quickly evacuated, barely managing to escape as Lenatix IV was wreathed in purple-blue flame.
While Chaos worship had not yet become prevalent on the forested Civilized World of Green Isle, apostasy was running rampant. Sensing a prime opportunity to spread the gifts of the Pox God, Plague Lord Lorphus Ebolion unleashed the terrible Curse of Unbelief upon Green Isle, taking advantage of the world's apostasy to let the Curse fester. Within days, the populace of Green Isle had been entirely transformed into a horde of foul plague zombies, and Ebolion deployed his Plague Marines to begin pumping the world's atmosphere with filth and disease.
In the aftermath of Oseonus III's fall, a shortage of water inevitably befell the Ulgroth Sector. While the Munitorum had approved the usage of emergency water reserves to sustain both the populace and war effort in the Ulgroth Sector, it would take time for those reserves to arrive. In the meantime, water would have to be rationed carefully, and those of least importance to the war effort would have to be left without a livable amount of water.
However, a method of lessening the number of lives that would have to be sacrificed could be found in the minor Fortress World of Frigoth Primus. A mountainous, tidally-locked planet just barely within the habitable zone of its system, Frigoth Primus was covered in naught but snow and ice. This snow and ice was plentiful enough to be harvested, melted, and purified into drinkable water that could be used to augment the existing supply. Unfortunately, Frigoth Primus was also one of the worlds attempting to secede from the Imperium, and its well-trained, well-equipped, and heavily militarized population was dug in around the planet's handful of well-defended fortress-cities. To make matters worse, Warsmith Savakai Moloth of the Iron Warriors also recognized the planet's value, and had sent troops, weapons, and supplies to the rebels in order to help keep Frigoth Primus out of loyalist hands.
Planetary defense guns made a direct orbital assault on the fortress-cities impossible, and thus loyalist forces would need to besiege them from the ground, which would require a several hundred kilometer trek to the target. The Cadian 207th Mechanized Infantry, 191st Shock Troopers, 77th Armored, and 22nd Siege regiments were deployed to make the journey. But the rebels used the planet's inhospitable conditions to their advantage, and morale among the deployed Guard forces fell quickly. In response, the Doom Eagles, Red Templars, and Sons of Eleusis Chapters deployed forces to aid the Guard regiments and hopefully improve morale.
Approaching fast and low by way of Thunderhawks, the Astartes struck at gun emplacements lining the Khine mountain pass, which the Guard had been trying for days without success to enter safely. With the pass cleared of rebel resistance, the Guard was able to reach the cities successfully and begin preparing for their assault on the first fortress-city: Vront Secundus.