Junthor

Description
Junthor is a large terrestrial planet with a thick atmosphere of carbon dioxide and chlorine. The surface is mainly composed of aluminum with deposits of nickel.

Surveyors found the ruins of a technical civilization near the equator – evidently the colony of an ancient spacefaring race. The ruins had subsided to almost nothing – merely wind hollowed husks of arcologies and other megastructures. In the center of the ruins was a single column whose inscriptions defied translation for several centuries.

When asari linguists finally managed a translation, the elaborate relief carvings said merely, "Walk among these works, and know our greatness." The crude scratches on the base of the reverse side said, "Monsters from the id."

Survey Text
''Scans revealed a wrecked freighter orbiting one of Junthor's moons. A recon team found no signs of life, but they did recover one of Matriarch Dilinaga's writings.''

Properties
Junthor is about half the distance from its sun as Mercury is from ours. Beyond that is the Rayingri / Vahtz double system. If Rayingri is temperate, for Junthor to have a stable orbit Junthor would have to be tidally locked to the sun and be very close to it; added to the carbon dioxide greenhouse, the temperature of this planet is unrealistic.

Also, chlorine is so reactive that there has to be something replenishing it in the atmosphere, possibly some kind of exotic flora.

Trivia

 * The phrase "monsters from the id" is a homage to the classic and hugely influential science fiction film Forbidden Planet, where an ancient civilization of unimaginable power gave birth to monsters from their own psyche (which according to Sigmund Freud's structural model of the psyche, is composed of id, ego and super-ego), and were wiped out by these said monsters.
 * The relief carvings on the column carry an echo of Percy Bysshe Shelly's poem Ozymandias:

And on the pedestal these words appear: "My name is Ozymandias, king of kings: Look on my works, ye Mighty, and despair!" Nothing beside remains: round the decay Of that colossal wreck, boundless and bare, The lone and level sands stretch far away.