A destroyed moon of Saturn . Previously the seat of the rebellious Moon Lords. The Ash Lord scorched it with a nuclear orbital bombardment, on the command of The Sovereign, Octavia au Lune, prior to the events of Red Rising. The Moon Lords' rebellion was caused when they did not accept Octavia au Lune as their new Sovereign. The memory and damage from the destruction of Rhea was never forgotten by the Rim.
“Who has bombed your cities? Who has slain a million of your people? Who kept your children hostage on Luna? Slaughtered your father and daughter on Mars? Who burned an entire moon? […] Your greatest enemy is the greed of the core. The burners of Rhea.” -- Darrow
The First Moon Lord Rebellion
The nuclear annihilation of Saturn's moon Rhea was the final event that ended the Moon Lord's uprising. At the time of its destruction, Rhea was the headquarters of the Rim's government. 60 million people inhabited Rhea when it was destroyed. The Ash Lord destroyed the moon with nuclear bombs from his orbiting fleet.
The memory and damage from the destruction of Rhea was never forgotten by the Rim. Darrow used the memory as leverage in his negotiations with the Moon Lord's after their second rebellion by rousing their memory of Rhea to convince them to ally with The Rising.
The Second Moon Lord Rebellion started 60 years and 211 days after the first.