Ionia is the name of the Rotakan moon, although it is more like a sister planet than a moon, especially give that it is slightly larger than Rotaka, not that anyone alive knows that. It is habitable, but only barely. It is much drier than Rotaka, entirely subsumed by vast deserts broken up by bottomless ravines and sky-scraping mountain ranges.
There is only one path to Ionia, the Worldgate, a massive gate situated in the heart of the Rotakan capital city. Soaring hundreds of paces in height, and sitting over a thousand paces in width, the gate to Ionia is the only permanent, self-sustaining portal in existence. No one knows who built it, and all attempts to study the gate have failed. Its very existence defies everything known about how graescence works.
That hasn't stopped the Empire from taking advantage of the gate, of course. Despite Ionia's deserts, the sands become surprisingly fertile when provided with water. This allowed the Empire to turn a massive caldera basin spanning over a thousand leagues desert into slave plantations producing enough food to feed most of the world. Moreover, the rock deep beneath the sands are riddled with precious minerals and metals that can only be found on Ionia.
Trillions of cubits of water are pumped through the gate into massive underground reservoirs beneath the Ionia desert to support agriculture and mining operations. Most of the water is then pumped up into millions of leagues of drip-feed piping for the agriculture above, with a smaller portion supporting the mining operations. What's left is rationed to the slaves.
Originally, the empire did this to keep its coffers full of money, but several hundred years into the empire's reign, it stopped selling the food and opted to give it away instead. The Royal family claimed to have solved the problem of poverty, freeing their citizens from the worry of food. At the same time, almost the entire agricultural industry collapsed, displacing millions of workers. This might have caused a revolt by itself, but food was now free and the empire offered special compensation for those workers to begin a new life. It was expensive, but it worked. At the same time, niche and specialty food prices skyrocketed as the industry shifted. Of course, slaves didn't need to be paid and while the transportation costs were borne by the empire, their control over the general populace became almost absolute. One did not bite the hand that fed it.
Meanwhile, the empire's strict laws kept a steady supply of slaves to Ionia. This was a bitter pill to swallow for many but the empire kept up an excellent propaganda program aimed at extolling the benefits to society. With the slaves literally out of sight, the empire having control of the food supply, and the general populace receiving these benefits, there was surprisingly little resistance. Eventually, it simply became an accepted way of life.
Ionia is essentially an unlivable planet with the only source of water pumped in by the Empire. The only food on the planet is produced by the slave plantations from both the above-ground agriculture and below-ground mushroom/moss farms.
Massive sandstorms are a common occurrence, and only the massive caldera ridges protect the caldera from them. These storms are so bad that anyone caught out in it is usually dead within a few minutes, their flesh sandblasted straight off their bone. The empire long since stopped trying to send expeditions to explore the desert.
Within the caldera, there's a large breadth of small desert animals. Snakes, lizards, scorpions, etc all share space with Ionians and slaves alike. Most of these can also be found on Rotaka, so it's presumed they migrated to the caldera through the Worldgate.
However, Ionia certainly used to support life. There is an abundance of evidence that Ionia once has oceans and rivers, along with all sorts of life. The fossilized remains of countless creatures have been found, along with local plant life deep within the caves that have no counterpart in Rotaka.
Furthermore, there are animals (or monsters, depending on your perspective) that live in the desert, having adapted to the harsh climate. The most famous of these is the Sandwhale, which can sometimes be seen breaching the sand dunes as though it were an ocean. There have been countless debates as to how these animals survive, but the most common belief is they don't actually live in the sand or even above ground at all. Instead, it's theorized that they must live in underground caverns uniquely suited to support life, perhaps similar to the way most Ionians live underground.
This has led many to believe that Ionia might one day be livable again if they could fix whatever caused the collapse of its ecosystem. How that could be done, though, nobody knows.