The Red Mountains were a mountain range on an unnamed planet that was home to a pre-Hebitian culture thousands of years prior to the 24th century. These people dug a series of tunnels under the Red Mountains, where they buried their royal dead. Also beneath the Red Mountains was the Well of Souls, where the dithparu were held, unable to escape without a humanoid host.
The name "Red Mountains" had been long forgotten by the time the burial chambers were rediscovered in 2336. (TLE novel: Well of Souls)