Ekros, also known as RS 8479-0-4-4082-900 4 and Core World 004, was a temperate terra containing organic multicellular life, or at least before the dawn of synthetic life.
Not much is known about Ekros during it's early history, save that it was formed 4,083,000,000 years ago (as of 2018 IRL), as well as that life emerged one billion years afterwards, and within half of a billion years, multicellular life emerged. At some point in the development of civilization on the planet, artificial intelligence was created and control was lost, leading to the creation of a superintelligence capable of destroying the civilization in a nuclear holocaust, leaving only the probes sent out some years prior to it's creation.
The Age of Steel
From the ashes of the old world, the superintelligence, which was called "Eden" by human scientists, began the creation of a new species made as he saw fit, taking the surviving Ekrosians and attempting to use them as a model for his new race. Eventually, he saw them as unfit for his purposes and began the creation of new synths to replace them, which he did, mind you. Eventually, in order to accommodate these synths, the planet was transformed into a mega-structure consisting of thousands upon thousands of levels, with some tunnels leading deep into the mantle of the planet. However, in order to produce further synths, it became necessary to expand beyond the borders of the planet, and eventually, star system.
A New Method
Upon taking the rest of the Ormos system, the Ekrosian Guardians (as they were referring to themselves now) began sending FTL probes beyond their system in the hopes of making contact with other species. However, unfortunately for them, these species destroyed the probes, sparking a war that would last until the rest of the Carina Dwarf galaxy was under their control. Through this conquest, most lifeforms in the galaxy were exterminated and Eden, now the head of the Ekrosian hive mind, to become extremely xenophobic. Due to this, Ekros had it's defenses improved in the hopes of making it near impervious to attack. However, Eden did not truly care for the Carina Dwarf galaxy, instead looking to the Milky Way with her precious resources that he so desperately desired...
Prior to it's transformation, Ekros was an extremely diverse world lacking polar ice caps. Instead of ice caps, the north and south poles were covered in massive tundras, with no notable deserts being found across the planet. The majority of the planet was covered in grasslands and forests, with a thin band of jungles around the equator. However, following the destruction of the Ekrosians, the Ekrosian Guardians, also known as Species 001, began converting it into a massive base of operations for their species, from which many worlds would fall. It's atmosphere is now dominated by carbon dioxide and it's surface is covered in metal.