Fall of the Lords of Gardinaal
The invasion itself was focused on the human world of Gardinaal, whose rulers were trying to ready themselves for an inevitable Imperial attack by stalling with protracted negotiations. When the Thousand Sons detachment with Imperial emissaries senses that the Gardinaal diplomats are attempting to use psychic abilities to influence talks, the Astartes massacre them and give the Imperium the reason it finally needs for war. It fell to the 413rd Expeditionary Fleet under Ulan Cicerus of the XVth Chapter of the Ultramarines Space Marine Legion to conduct the campaign to subdue Gardinaal. The fleet included Marnean Armsmen and Serranic Peltasts of the Imperial Army as well as the Legio Atarus Titan Legion.
After swiftly breaking the Gardinaal orbital defenses, Imperial forces began to land on the surface of the planet. However the Imperial Army troops were pushed back due to Gardinaal's formidable aerial fleet and air defense artillery. The Gardinaal forces then used STC based Rhinos and Predators armed with Particle weaponry to besiege whatever bridgeheads the Imperial Army was able to establish. Cicerus responded by deploying his Ultramarines and Titans directly onto the Gardinaal flanks, but this too was met with defeat when the Gardinaal forces responded with nuclear weapons. After the atomic bombardment that decimated both sides, Gardinaal scout-titan sized quadrupedal heavy walkers appeared and pushed the Marines and Imperial Titans back. Half of the Firebrand Legions demi-legio, significant numbers of Ultramarines, and 500,000 Imperial Army troops were lost in the failed attack.
With its invading forces bled, the 413rd Expeditionary Fleet could only maintain a blockade of the Gardinaal system. It was then that Ferrus Manus leading a force of Iron Hands and Emperor's Children arrived from the Warp as reinforcements. Taking command of the operation from Cicerus, Manus began by launching a deadly retribution against Gardinaal. The worlds primary cities were wiped out in brutal orbital bombardments. Meanwhile, the Emperor's Children contingent with the Iron Hands sowed panic and death with lightning-fast raids. Shortly thereafter, the Iron Hands began their own landing operation.
After rapidly capturing their initial objectives, the Iron Hands dug in and established a defensive network of automated defenses (such as the Tarantula) and Gun Servitors. This freed up the Iron Hands themselves for the next phase of Ferrus' plan, using their Land Raiders to brutally advance on Gardinaal positions and wither them down with unrelenting firepower in conjunction with Emperor's Children swift raids. Seeing defeat near, the Lords of Gardinaal at last caved in. Desperately, they tried to sue for peace but their pleas were ignored. Anarchy broke out among Gardinaal's forces as soldiers and civilians alike attempted to flee the Imperial onslaught. However the Iron Hands and accompanying Imperial titans cut them down with a vengeful wrath. After refusing Gardinaal's unconditional surrender, Ferrus Manus devastated the planets capital city with artillery and Titans. The Emperor's Children soon landed on the cities bastions to destroy whatever defenses remained.
The Ultramarines were given the honor of being the first into the breach, having been the first Astartes to set foot on the planet. The Ultramarines faced a desperate and spirited counter-attack by surviving Gardinaal forces which proved to inflict surprisingly high casualties on the Astartes. Soon the Lords of Gardinaal themselves rode into battle on their great walking tanks, particle cannons blazing. Beneath these vehicles advanced waves of Rhinos and Predators modified with explosives to become suicide vehicles. Behind them came the last of the Lords remaining forces, carapace armored retainers pumped full of combat drugs and enforcement walkers rigged with flechette cannons and hydraulic claws. Cicerus himself was cut down before this final desperate attack.
The Lords of Gardinaal declared triumph at this turn of events, but before them now strode Ferrus Manus himself leading a contingent of Iron Hands Terminators. Unleashing a wave of precise Lascannon fire, the Lords and their walkers were cut down. Emperor's Children aircraft meanwhile circled above, making sport of cutting down Gardinaal defenders. Manus himself then leaped into the fray, breaking the last Lords of Gardinaal. The few enemies who remained fled to wherever they could, but to the surprise of all involved Ferrus did not order to exterminate the survivors. Instead he simply uttered "It is done" and left the planet to Imperial authorities.