Emancipation of Drune
The campaign was waged at Drune, a world that had once been a beacon of civilization. The Imperial invasion was led by three Legions - the Sons of Horus, Death Guard, and White Scars as well as their respective Primarch's. However upon making planetfall, there was no reaction from the local population. The Stormseer Kulek Senn suspected something was amiss and urged caution the Primarch's Horus, Jaghatai Khan, and Mortarion.
As they pressed onwards into the largest city of Drune, a great ragged mass of humanity was found on the streets. Every body was unwashed and stinking, each body a drooling simpleton. Once more Senn urged caution, drawing the sneer of the distrustful Mortarion. However at this moment the skies opened up and an unnatural energy washed over the city, stirring the population which began to groan as once. Retreating from the city, the Primarch's determined that the world was under some sort of Xenos domination. It was determined that xenos warp-portals centered upon the major mesa-top cities were exerting control over the population. Declaring that they would liberate the local humans from the xenos, Horus made Jaghatai Khan the campaign commander despite Mortarion's protest. Horus kept the Death Guard away from the Scars to avoid issues, but the Imperials nonetheless waged an effective campaign. For six months the Great Khan organized scouting ventures across Drune, striking at concentrations of xenos power identified by his Stormseers. During these raids the Scars encountered a range of horrifying bloated creatures. These tendriled multi-eyed creatures could unleash waves of warp fire as they commanded legions of enslaved humans at the Imperials.
At last the campaign reached its climax at the same city that the Primarch's had first ventured into. The closer the Primarch's advanced, the more potent the psychic waves assaulting them grew. The Khan used his Stormseers to protect the Imperials as they advanced as the Primarch's reached the center of the city. They came before an archway of quivering living flesh which served as a gateway to the Warp. From this portal emerged a vast and bloated form with thrashing pseudopods and dozens of eyes. The Khan flew at this foe, his brothers following him into battle while the Stormseers held off the creatures vile kin. The creatures psychic power was nonetheless able to kill 10 Stormseers in an instant as he held three Primarch's at bay. Horus and Mortarion reeled under the creatures psychic assault, but the Khan was able to defeat the beast by using the last of his Stormseers to render himself invisible to the behemoth. Locating a weakpoint in its underside, the Khan delivered the killing strike. As the creature died a warp vortex opened up that would have consumed the Khan had it not been for Horus. As the portal closed the last of the Xenos vanished or died, while their mind puppets collapsed and never stood again. This was in truth a mercy, as these poor souls could not have been allowed to live under Imperial rule.