Conquest of Galaspar
Waged against Galaspar, the capital Hive World of a pocket empire, the human civilization there had refused to accept the rule of the Imperium and opened fire on Explorator vessels and Imperial envoys. It eventually fell to Mortarion and his Death Guard to force the Galaspar Cluster into compliance. So it was without warning that the Death Guard fleet smashes through the outer defenses of the Galaspar system.
Defeating the initial enemy defenses of asteroid-fortresses and Fire Ships, Mortarion's fleet made a direct course for Galaspar itself, ignoring the surrounding planets. Reaching Galaspar's orbit, Mortarion led the invasion of the world his personal assault barque, the Forth Horseman. The three kilometer-long craft plowed into the upper Hive's of Galaspar, toppling the cities spires and unleashing its deadly cargo of 11,000 Death Guard Space Marines. Galaspar's primary hive fell in a night and a day, the Death Guard killing any man, woman, or child who stood before them.
The unprepared slave-soldiers of Galaspar found themselves reeling in terror at the Astartes onslaught. Falling back to defensive redoubts, the Galaspar forces Stubbers and Lasguns proved useless against thick Astartes Power Armor. The Death Guard seized ground, destroyed enemy counter attacks, and advanced in its wake in a cycle that broke the Galaspar defenders. The Death also employed biological weapons, using Phosphex Bombs to sow death and terror. The enemy forces began a bloody retreat into the lower depths of Galaspar's hives, but even in this noxious and dark environment the Death Guard found and slaughtered them. With Galaspar's primary hive in firm Death Guard control, the Legion's Techmarines took control of the main planetary orbital defenses, quickly disabling them. This allowed the orbiting Death Guard fleet to safely land even more reinforcements on the world.
However the rulers of Galaspar were not yet defeated, and the worlds secondary hives spewed forth tens of millions of soldiers and thousands of tanks. The Death Guard however met the enormous attack, using their orbiting warships to unleash devastating bombardments on the tightly massed enemy forces. With the bulk of Galaspar's remaining forces destroyed in the open, each of the worlds secondary hives were methodically encircled, besieged, and violently captured, the Death Guard ignoring any proclamations of surrender. Mortarion showed no mercy, dispatching forces to pursue and hunt down the scattered remains of fleeing enemy forces in the later stages of the battle. It is said that not one of the enemy defenders survived the battle.
When Imperial support fleets arrived to take control of the planet, they were horrified at the slaughter and destruction the Death Guard had unleashed on Galaspar. However the people of Galaspar, used to the oppression of their previous rulers, meekly accepted Imperial rule. Moreover with the example Mortarion had made on Galaspar, every other world in the surrounding systems quickly submitted to the Imperium, lest they suffer the wrath of the Death Guard.