Otherworld is the name given to the planet ruled by Overlord that has been the source of multiple invasion attempts against Earth. The exact location of Otherworld, or even what dimension it exists in, is unknown. It is possible that it exists in the same dimension as Earth, but if so it's location is so far across the universe it makes travel there by known means of space flight impossible. So far all travel to and from Otherworld has been by crossing through other dimensions.
What is known of Otherworld is that it is larger than Earth, but has a comparable gravity and atmosphere. It orbits a Red Giant sun that is part of a binary star system. Otherworld's sun's companion is a White Dwarf star that orbits far out from the planetary system. At one point in its history, Otherworld had a natural moon, but it was destroyed by Overlord when he conquered the planet. All that remains of the moon is an asteroid ring circling the planet.
There is no written history of Otherworld. The true details of the world's past are likely known only to Overlord and a few of his most trusted Elite. Some facts can be discerned, however, by finding common themes in the legends of the numerous cultures, or at least in the human-derived ones.
Other items have been gleaned from the few records that have been salvaged from buried ruins in the Great Desert (apparently the only ruins to remain intact after Overlord's invasion). It is known that the original populace had formed an essentially unified central government and had reached a technological level on par with late 20th century Earth. They were more advanced in some areas, like biotechnology, but less advanced in engineering fields. For example, they were only just beginning a space program to explore their solar system and probably had almost no warning of Overlord's arrival. It is known from the surviving records that Overlord transmitted an ultimatum over all broadcast systems demanding the planet surrender to him. There was surprisingly little deliberation before the planet issued its response, and doomed itself. With only a nascent space program, the people had no real defense against Overlord, but chose to oppose him nonetheless. It is at this point that records become essentially useless as no electronic documents have been found intact, indicating they were destroyed by an electro-magnetic pulse or similar phenomenon, and surviving paper records are scarce and often indecipherable even when found. Whatever the form of Overlord's attack had been, it was swift and devastating. It was likely at this point that Overlord destroyed the planet's moon, but all that is really known is that once the dust had settled the planet's people were scattered and their civilization thrust back into the Stone Age.
Overlord has likely reigned over Otherworld for thousands of years, but again only he knows for sure. After the Cataclysm the scattered pockets of civilization began to rebuild, sometimes managing to salvage old technology from the time before Overlord, other times progressing from scratch. Eventually some areas evolved back to a level where they would be useful to Overlord. At this point he would either bring them into his Empire or enlsave them. Cultures that swear allegiance to Overlord are required to pay a regular tithe in the form of goods, raw resources, or manpower; and Overlord either sets one of his lieutenants as ruler there or promotes one of the culture's own representatives to the position.
Generally speaking, Otherworld is not a nice place to live. After the Cataclysm, all of the cities were destroyed and much of the planet reverted to wilderness. Whether by evolution or by Overlord's manipulation the wilderness was harsher than before. Each of the scattered remnants of civilization found themselves surrounded by a hostile environment. The result was that many of the groups of survivors were wiped out by the new creatures that prowled the forests, mountains, plains, and rivers. Those that survived did so by being stronger and more aggressive than their environment. The entire planet had become a lab for Darwinian theory. Eventually the pockets of civilization began to rediscover each other, which usually led to confict.
Today the planet is still mostly wilderness, broken by pockets of civilization where groups have rebuilt and expanded. Each area of civilization seems to have been deliberately surrounded by harsh wilderness with dangerous creatures. Presumably this was an effort on Overlord's part to promote survival of the fittest amongst the surviving populace. This also helped accelerate the divergent evolution of the survivors. Today there are dozens of distinct evolutionary branches derived from the planet's original populace. And now whole new intelligent species are being discovered that have evolved from the non-sentient indigenous animals.
The physical after-effects of the Cataclysm are still being felt across the planet as well. There are large holes in the ozone layer at the poles which let in a great deal of radiation, making the poles a dangerous place for humanoid life, and the asteroid ring constantly bombards sections of the planet with meteor showers.
There is really only one authority on Otherworld, Overlord. The populace is divided into kingdoms which have evolved from the scattered remnants of the original civilization. Any kingdom that grows large or advanced enough to attract Overlord's attention is approached and ordered to swear loyalty to Overlord. Those that refuse are conquered or wiped out. When a kingdom swears loyalty to Overlord or is conquered, a puppet ruler is put in place to govern in Overlord's name. If there is no one suitable amongst the local population then Overlord appoints one of his lieutenants to the position. If a local is selected then that individual is often augmented by Styxx to make them more powerful than most of their subjects to ensure that person will be able to stay in power. Each kingdom is left to govern itself as long as it sends a regular tithe to the Citadel. The tithe can be raw or finished materials, goods, or even a steady stream of slave laborers. Overlord does nothing to prevent kingdoms from waging war on each other, in his view it helps weed out the weak. As long as he gets his tithe Overlord seems content to leave the kingdoms alone.
Overlord does seem interested in evolution and the concept of survival of the fittest. Because of the extra background radiation and Overlord's machinations many of the species on the planet are beginning to show random mutations. There is a standing order in all kingdoms that any children with radical mutations be handed over to the Citadel for examination. The mutants are placed in Styxx's lab where he determines the nature and possibly the origin of the mutation. Styxx's methods of examination, of course, include no concern for the welfare of the subject. A group of mutant test subjects escaped from Styxx's lab several years ago and have formed the core of a Rebellion against Overlord.