The Cthonians are a long-dead alien race that inhabited the planet Icarus and visited Earth on several occasions, influencing Greek mythology. They were notable for the use (and possibly the creation of) bio-metal, a semi-metallic substance used extensively in the Biometal Wars.
The Cthonians themselves were almost perpetually at war; their race was split into two factions, the Olympians and Hadeans, who each occupied one of the planet's larger continents. Their previously ceaseless war ended abruptly with the arrival of the Furies, purely biometal craft which almost destroyed both factions. The Cthonians united to destroy the Fury abomination, but in doing so sacrificed both themselves and their planet, which now makes up the asteroid belt between Mars and Jupiter.
The Cthonians had influence across the solar system, as seen by the presence of relatively intact Cthonian relics on Mars and the moons of Jupiter and Saturn. Whilst it may be possible that some Cthonians survived the cataclysm that destroyed their planet, this is increasingly unlikely given that the event occurred some two thousand years ago.