Xyan is a heavily sulphuric Penal World found within the Orpheus Salient of the Jericho Reach, though its native Human population, being descendants of criminals, has shown only reluctant cooperation with the Imperium.
Xyan has a brethable but highly acidic atmosphere which slowly damages the lungs of those condemned to live upon it.
Those few million Humans who have dwelt on the planet have done so in small communities, typically of less than a hundred members. Through the ages, they have adapted to their toxic environment, and can tolerate the highly sulphuric atmosphere and diet far better than an outsider. Their vision has also adapted to the haze, though not so well that they would generally be classified as mutants.
During the time when the Jericho Reach was still the Jericho Sector, Xyan became a Penal World. Heretics, criminals, and cowards were dumped on the planet's surface, where survival of the fittest was allowed to run its course. Little was done to monitor the system, as the hazardous environs were deemed sufficient punishment and an effective prison.
Millennia later, the planet's population are the offspring of those who managed to survive the brutal conditions. Largely amoral filth, the natives have not responded well to the Achilus Crusade's efforts to reunite their planet with the Imperium of Man.
Some of the crusade's officers argued that, as descendants of prisoners, these inhabitants could scarcely qualify for membership in the Imperium. Of course, as a Penal World, they were not due any support from the Imperium either.
This combination meant that reasserting compliance, after once more designating Xyan a Penal World, was an easy feat. The Orpheus Salient has already begun the process of raising Penal Legions from the system's hardy population. These descendants of Imperial outcasts have co-operated reluctantly, as they have little recourse.
As units of the Departmento Munitorum have begun to spend time on the planet to train the first regiment, they have come across a number of worrisome legends. It seems that there are many tales of aggressive xenos that lurk in the hottest and murkiest parts of the swamp.
While there is no evidence to confirm this, there are always disappearances. The atmosphere makes tracking anything difficult, which has led to numerous unexplained disappearances among natives and soldiers of the crusade. Even establishing stable outposts on the murky surface has been a challenge for the Imperial units.
Xyan's atmosphere is tinged yellow by the sulphurous fumes that constantly bubble from the hot springs scattered across its surface. These dense fumes leave the planet's surface a dim place, as the thick acidic clouds diffuse light.
While breathable, the atmosphere leaves a foul taste in the mouths of those who visit, and lasting scar tissue in the lungs of those who stay. The planet's highly active core causes frequent tectonic instability, which is likely tied to the constant eruptions from the assorted geysers and the steady emission of gases through the hot swamps.
The planet's ubiquitous swamps are filled with aquatic life that thrives in the intensely acidic environment. Plant analogues draw energy from the geothermal heat that flows through these swamps, using this as an energy source rather than the light of the system's star.
Small predatory animals are common throughout these bogs, but they represent far less of a threat than the atmosphere and the sulphuric pollutants.
- Deathwatch: The Achilus Assault (RPG), pg. 60