Kastun is the name in imperial for what is, almost as much as the sun, the dominant celestial body in the sky of avishraa.

The fourth planet of the star Mikurmiya, it is a Class II gas giant slightly larger than Saturn. In visible light the planet's surface appearance consists of bands of white and an only slightly contrasting off-white, pale bluish-gray color; being both large and highly reflective, it is not only the brightest object in Avishraa's sky but also one of the brightest in the Mikurmiya system as a whole.

Kastun has at least thirty-five moons, but only three of them are more than 200 km in diameter; of these, the outermost, and by far the largest, is Avishraa, the only one of the moons known to bear life.

• Periapsis: 126735500 km
• Apoapsis: 139627500 km
• Eccentricity: 0.0484
• Orbital period: 307.56 Earth days
• Inclination: 0° to ecliptic

• Equatorial radius: 62268 km
• Polar radius: 58364 km
• Flattening: 0.0627
• Surface area: 4.94136 x 10¹⁰ km²
• Volume: 9.47902 x 10¹⁴ km³
• Mass: 6.68460 x 10²⁹ g
• Mean density: 0.705199 g/cm³
• Equatorial surface gravity: 11.506 m/s²
• Escape velocity: 37.85 km/s
• Sidereal rotation period: 9.9244 hours
• Solar rotation period: 9.9392 hours
• Axial tilt: 3.81° from orbital plane

The orbit of Kastun around the sun has a semi-major axis of slightly over 124 million kilometers, placing it and all its moons within the system's habitable zone.

Kastun rotates quickly, such that it exhibits notable flattening along its axis.

While the planet has a comparatively tiny, nickel-iron core, the bulk of its mass is composed of hydrogen and helium in various phases.

As with its interior, Kastun's deep atmosphere is composed mainly of hydrogen and helium. There are, however, traces of other gases, particularly methane, ammonia, nitrogen, carbon dioxide, and water vapor, the latter of which is responsible for the bright white clouds that dominate the planet's visual appearance. As a result, there are layers in the upper atmosphere where something like a simplified version of a terrestrial water cycle exists, with water rain drizzling down into higher-temperature layers and evaporating before eventually being carried back up to the cloud layer.

Life exists on Kastun, mostly in the upper layers of the atmosphere within or near the regions where water vapor exists in large quantities. The biosphere consists almost entirely of unicellular life, mainly a complex of anaerobic methanotrophs and nitrifying bacteria. Given their similarities to equivalent species on Avishraa, the closeness of the latter, and the unlikelihood of such organisms arising from abiogenesis on Kastun itself, it is most likely that Kastunian life originated from organisms transported from Avishraa, possibly deposited into the atmosphere via debris from a meteorite impact.

As Avishraa is tidally locked to its parent, Kastun remains in a stable position in the moon's sky, and shifts only slightly on a seasonal basis from the point of view of an observer; in addition, it is more or less visible, absent cloud cover, at all times of local day on the planet-facing side. As such, it is a commonly used point of reference in celestial navigation on the side of Avishraa facing the planet.

By itself, it provides an estimate of distance to the far northern heartlands where Kastun is nearest to zenith; in conjunction with observation of the stars and the sun, it can be used to determine one's position nearly anywhere on the planet-facing side. In many unurbanized cultures among both takmar and xtauh, among the lore taught to the young are wayfinding chants, describing prominent stars in the Avishraan sky for navigation at night, how to determine the motion of the sun during the day, the meanings of the relative positions of those inner moons visible from Avishraa, and the relationship of each to the position of Kastun itself.

On the far side, from which Kastun cannot be seen, purely celestial navigation is complicated by the moon's lengthy local day; using sunrise or sunset to observe the relative positions of the sun and stars is only feasible every few Earth weeks. As there are no known settlements of sapient beings on the far side of Avishraa, the science remains relatively undeveloped.

In the imperial_religion, Kastun is the home of the siathar, from which they watch over and protect takmakind.

In the mythological complex referred to as eyes_of_the_sky, prevalent among xtauh tribes of the central brightness, Kastun is described as being one of the eyes of daxelh, the creator goddess.