Tartarus

A struggling planet further ravaged by the elite who live in verdurous bliss whilst everyone else suffers an infernal landscape. Scattered across its surface are remnants of an advanced, predecessor civilization—much of which remains a mystery to the current, mostly-medieval society. Fortunately, these ruins are understood just enough to serve as oases amidst the inhospitable terrain. Much less fortunately, to be exiled from a habitable zone is to be sentenced to death. In short, whereas Arcadia is utopia, Tartarus is dystopia. 

Strewn across Arcadia's landscape are clusters of state-sized, climate-controlled domes connected via underground tunnels. These structures, erected by the absent settlers, have been maintained ever since by tireless automatons, and it is only thanks to these machines that intelligent life had the room to grow at all. Said creatures, named Machina by their creators, only ever fulfill the duties commanded of them long ago; their contemporary guardians have yet to decipher their programming. The Machina are given the utmost possible care by Tartareans.

Supplies on Tartarus are limited, with much of the procurement coming from scavenging ruins (scrap metal for armor, etc.) or force (wars for dome territory, etc.). Natural supplies are likewise scarce, with water tough to procure (the condensation system in domes are a high priority) and the plantlife all but inedible. Those living on the outskirts of domes face the worst of these conditions and must endeavor to remain hidden from the heartless aristocracy hunting them for sport. With the basic needs of the higher-ups being met, they instead place incredible value on attaining fuel and see it as a mark of power, often hiring mercenaries or placing bounties to attain their desires. Arenas with alluring rewards entice the desperate to enter; above all, these grounds serve to entertain the elite. 

The atmosphere on Tartarus is red, although its inhabitants are all but unaware of this; all domes generate an artificial, Tellus-like sky. 

Placement-wise, Tartarus to Arcadia is what Venus is to Earth; it is closest to Pyr among the habitable planets. Much of the intelligent life on Tartarus evolved from the livestock left behind by its former, humanlike settlers, and thus bear features similar to farm animals (pigs, cows, etc.). Said beings were forced to leave the idyllic Arcadia and survive on Tartarus for reasons unknown...