Sivvos is the second of four remaining planets in the Chirasheh system. Its sun Chirasheh, once a yellow dwarf about 1.5 times the mass of Earth's sun, has entered its red giant phase. Two billion years ago, Sivvos was probably a frozen world, possibly with a thick nitrogen-methane atmosphere similar to Saturn's moon Titan. But as Chirasheh expanded, much of Sivvos' original atmosphere was probably blown away by solar winds. The Valposhi, who evolved on Sivvos, have recently discovered objects that many believe are artifacts of an earlier civilization, possibly one that evolved on an inner world that was devoured by Chirasheh as it expanded. The fate of such a species is unknown.

Sivvos is a world of deep cracks and chasms, many of which were created as the frozen, icy surface of the world was baked by the increased radiance of the expanding star. The surface of Sivvos has only a very thin atmosphere, similar to that of Mars, but rather than a frozen desert, the surface of Sivvos is hot, averaging just below the boiling point of water. However, the deep cracks in Sivvos' surface have allowed for the formation of a thick, cool, soupy atmosphere of nitrogen, oxygen, and water vapor that increases in density as one descends into the deepest parts of the chasms. It is in these deep fissures that life on Sivvos evolved.

Sivvos has no tectonic activity, and only scattered and occasional volcanism. The fissures in its surface are believed to have formed quite rapidly as its surface was baked and its original atmosphere blown away by Chirasheh's expansion. Sivvos has a large iron core, which produces a strong magnetosphere. This high iron content is unusual for a planet orbiting at Sivvos' distance from its sun. There are two leading theories about why this would be. The first suggests that Sivvos originally formed much closer to Chirasheh and was pulled into its current orbit through gravitational interactions with the system's two large and distant ice giants. The second contends that Sivvos itself was once an ice giant, but lost a great deal of its gas envelope and mass when Chirasheh expanded, leaving little more than its core.

Although Sivvos is roughly the size of earth, it is more dense, resulting in approximately twice earth's gravity at the surface. The day on Sivvos is around fifteen hours long. Sivvos orbits Chirasheh once every 140 years. It has a very slight axial tilt, meaning that at the surface very gradual seasons are detectable, but in the depths of Sivvos' canyons, the environment is quite stable throughout the long Sivvosi year. The strong winds in the upper regions of the chasms spread warm air throughout, though near the poles, the water in the deeper regions freezes seasonally.

The environmental conditions on Sivvos change drastically between the surface and the depths of the canyons. The scorched rims of the Chiruss region are jagged and tend to have smooth, sometimes almost glassy walls. With only a very thin atmosphere, these walls have remained largely unchanged for hundreds of millions of years. A few extremophile species of single-celled life form lichen-like mats, but little other life exists in these conditions.

Farther down, Strong winds are created by the thin, dry heat from above mingling with the cooler, damper air below. The rocks in this region have been carved into a variety of dramatic shapes by this constant wind. Tough, shrubby photosynthetic life-forms cling to the walls here, and many species of arthropod-like organisms exist.

The Halthuss region is a realm of increasingly dense fog, where water condenses on the cool rocks and forms many small streams. These streams sometimes form covered caverns hundreds of miles long, which extend from the walls of the chasms in twisting, rootlike patterns. This region contains the greatest biodiversity on Sivvos, and many cave systems, isolated from others for millions of years, contain both terrestrial and aquatic species found nowhere else. Since their encounter with the Chimera of Fear, many Valposhi settlements have moved to this region, which is brighter than the Flithuss, where the Valposh evolved. This region is the most Earth-like on Sivvos.

The Dark Flithuss region has a heavy oxygen-water vapor atmosphere much denser than that on Earth. In most places, the upper extent of this region was once the base of the chasms, but millions of years of flowing water has carved ever deeper into the crust. This is the home of Sivvos' largest organisms, from sessile, fungus-like organisms to the centipede-like burrowing boloros, which can reach lengths of 30 meters. Some species make use of bioluminescence, and many, including the Valposh, have specialized eyes that see in the infrared spectrum or even longer wavelengths of light, which more easily penetrates the thick dark fog of this region. Many other species are completely blind, using feelers to detect movement.

The Douluss region consists of deep rivers and lakes created by condensed fog and water that has flowed down from the Halthuss. Continually deepening the canyons and carving them into bizarre and twisting shapes, the Douluss is a harsh but nutrient-rich environment that is home to a multitude of swimming and bottom-crawling creatures. This is the coldest and most stable region of the chasms, with temperatures usually hovering just above the freezing point of water. Many species here are very long-lived (hundreds of earth-years in some cases) and make slow migrations from pole to tropics and back over the course of the long Sivvosi year.

Avan the Brightling