Location from Red Rising by Pierce Brown
Sun-scorched warfront where the Republic's forces became trapped in a grinding siege. The Ash Rain falls here — volcanic debris that chokes the sky and buries the dead.
Mercury is where the Rising's idealism went to die. After the Republic formed, Darrow took the fleet to Mercury to break the remaining Society loyalists. What should have been a decisive campaign became a quagmire. Atlas au Raa's guerrilla tactics, the brutal terrain, and the Ash Rain ground the Republic's best legions into exhaustion. The planet itself fights you — the temperature differentials, the volcanic activity, the choking ash that fouls equipment and lungs alike. Darrow spent years here, winning battles and losing the war of attrition, while back on Luna and Mars the Republic he built began to fracture without him. Mercury proved that you can win every fight and still lose everything. The Ash Rain buries the living alongside the dead, and after enough years, you cannot tell the difference.
A hellscape of extremes — the sun-facing side is molten rock and radiation, the dark side frozen waste. The habitable band between them is a narrow strip of fortified cities, military installations, and ash-covered battlefields. Volcanic vents spew debris that falls as the infamous Ash Rain, coating everything in grey-black particulate.
Also known as: Mercury, the Ash Rain front, the Mercury campaign