Herisor (Death World)

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Herisoris a Death World within the Achilus Crusade's Orpheus Salient. At first glance, Herisoris a Paradise World that appears lush and ripe for humanity's incursion. The planet has a broad temperate zone, with temperatures that are ideally suited to human life. The atmosphere is oxygen-rich, the gravity is close to that of Holy Terra, and the seas and lands have lush vegetation and abundant, herbivorous fauna. No predators or parasites that target humankind have been identified. Herisoris seemingly perfect for human habitation.

But appearances can be deceiving: something is terribly wrong on Herisor. Unprotected humans will become deathly ill within weeks or months of setting foot on the planet. The cell structure starts breaking down as if affected by an accelerated aging process. No explanation for the phenomenon has ever been found. Wear protective gear is only somewhat effective; sooner or later those who spend time on the surface will wither or die, protection or no. Not even Deathwatch Astartes in full armor are immune.

Herisor has shown great resilience against the Tyranids of Hive Fleet Dagon, whose forces grow sluggish within the planet's atmosphere, then lethargic, before finally succumbing to the same unseen force that kills humans.