Mila Xa was a memorial and monument to Saeena, bride of the Emperor Muro III. It was a jewel-encrusted mausoleum that built by Muro III's robot army of 20,000. It was built in the 17th century on an asteroid. It was seen by the Federation as one of the most astounding memorials ever built by one man's love.
After Muro staged his bride's death, he had a royal hideaway with an artificial atmosphere that had a "cottage" torn down. He then had his robot army carve it into a breathtaking mausoleum. Muro had fabulous trees, flowers and forest creatures brought there from all his empire. It was built with an mammoth computer to maintain the environment and to create ghostly illusions. It was seen as a near paradise.
In truth however, Mila Xa was built as place to keep Saeena hidden due to her mutation, where Muro III would join her until death. He joined her after he disappeared on a space expedition. However, Muro died at 84. The memorial then became his, not Saeena's.
In the six hundred years since it was built, hundreds of thieves tried to sack Mila Xa of its gold and gems. All thieves disappeared, except for one. However this one thief then went mad. These disappearances caused ghost stories to come about. Thus in 2266, the Federation dispatched an investigator, Jay Nordyke, to end these ghost stories. Nordyke then spent four days. The USS Enterprise under Captain James T. Kirk was dispatched to rendezvous with Nordyke and explore Mila Xa. (TOS comic: "The Haunted Asteroid")
In a letter to the commander of Starfleet Archives Records Section, Kirk recalled his experiences at Mila Xa. Though it felt terrifying at the time, especially when seeing apparitions, in retrospect he considered it to be more amusing. He was amazed that Saeena had lived alone in a mausoleum for centuries. (The Enterprise Logs, Volume 3)