Elidan IV is the third planet in the Elidan system in the inner Perseus Arm. A minor Federal colony world, Elidan IV is the only habitable planet in its system. It is a rocky terrestrial world with famously high volcanism, an greenhouse atmosphere, and a sparse, arid biosphere dominated by mangrove swamps and salt marshes. Acidic oceans cover around 48% of the planet, but prior to major volcanic activity, it was considerably more and composed of less-acidic water. First colonized by humanity in the early 2500s, a colony flourished for almost three hundred years. Prior to human settlement, Elidan IV had evolved a highly diverse native ecology with flora and fauna comparable to that of Permian-era Earth.

From 2790 to 2791, a series of massive caldera eruptions destroyed the native ecology, annihilated the human settlements, and drove most of the colonists off-world to settle elsewhere. A volcanic winter persisted for eight more years, with roughly a century of "volcanic autumn". However, continued volcanic activity combined with industrial pollution greatly thickened the atmosphere and led to a runaway desertification effect. Most of the world is presently very arid, with low ocean cover and much of its land covered in deserts or acidic wetlands. The volcanic events on Elidan IV are considered one of the greatest ecological catastrophes and natural disasters in human history. Over a quarter of the native fauna species went extinct, mostly in the oceans; much of the woody flora perished; glaciation accelerated, but then rapidly reversed. Around a sixth of the ocean cover retreated. Various explanations have been given for this phenomenon: in shallow, coastal regions, the ocean retreated due to extreme salination; in volcanic sea-lakes, water was ejected into the atmosphere in the eruptions; in the post-catastrophe desertification, considerable amounts of water evaporated in the oppressive heat. Over a hundred million people died in the immediate eruptions, out of a population of 448 million. Over two hundred million emigrated to other worlds over the next century. The remaining population of 150 million was halved over the following four hundred years.

The planet's remaining population was plunged into poverty and deprivation due to the ecological and climate effects of the eruptions. The modern population of Elidan IV has some of the lowest life expectancy of any human colony, and consistently ranks among the economically poorest worlds. It produces no significant exports, and relies on tourism and industrial resource extraction to sustain its economy.

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