Much of the planet's history is unknown; however, it is known that droids were sent to the planet to develop its resources and prepare for colonization. The droid in charge of this operation was an intelligence named M4-78 and was assisted in the colonization effort by two additional intelligences named IS-24, an industrial Archon, and ES-05, an environmental Archon. During the colonization effort, the Sith arrived and claimed that they were the original colonists. Programmed to obey the "colonists" upon their arrival, M4-78 ordered the droids of the planet to serve the Sith in any way necessary. The Sith, after discovering the planet's natural resources and the droids' ability to harvest and refine them, ordered IS-24 to begin production of a massive droid army. To conserve resources and energy, the Sith also shut down M4-78.
Eventually, Master Lonna Vash and her apprentice Kaah Ohtok came to M4-78 in hopes of ending the Sith occupation. Kaah, being an expert with computers, was able to slice into the colony's environmental systems, where he and Vash were eventually able to convince ES-05 that the Sith were not the colonists they claimed to be. Kaah and ES-05 devised a plan to flood the colony with radiation, killing all the Sith on the planet, leaving only droids.
Before their deaths, however, the Sith were able to do two things. First, they sabotaged the connection between IS-24 and ES-05, preventing the two Archons from easily reactivating M4-78. Second, a few Sith were able to discover and attack Kaah and Vash's position. Vash was knocked unconscious and the Master and Padawan were separated. Upon losing his connection to Vash, Kaah believed her dead, which led him to take up a misguided quest to prevent any Sith from retaking M4-78.
Upon arriving on M4-78 only droid party members could explore the planet until they were able to clear the radiation that Kaah and ES-05 had released. When the Jedi Exile was finally able to explore parts of M4-78, she met IS-24, who told her that in order for M4-78 to be reactivated, ES-05 also had to agree. As the Exile made her way through the environmental zone to speak with ES-05 she was contacted by Kaah who was impersonating ES-05. Kaah accused the Exile of being a Sith Lord and refused to listen to anything the Exile had to say. He sent several waves of droids to attack her, in an attempt to prevent what was, in his mind, the beginning of another Sith occupation. Eventually the Exile was able to reach Kaah where his true identity as Vash's apprentice was revealed. The Exile then had the option of helping to guide him back to the light, killing him, or driving him to the point where he killed himself. With Kaah finally out of the way the Exile was able to speak with the real ES-05 who agreed to reactivate M4-78.
With M4-78 activated the Exile was able to enter the Main Behavior Core where M4-78 was housed. It was here that the Exile was to finally meet up with Vash. Vash told the Exile that she had been rescued after her battle with the Sith by L5-65, one of the droids the Exile encountered patrolling M4-78's Central Zone. L5-65 had brought her to M4-78 and she had been locked there ever since. The Exile had the option of killing Vash here or agreeing to meet her on Dantooine with the rest of the Jedi Masters.
Darth Sion, who discovered the Exile's location, went to the system and seemingly killed the Exile. On the Ebon Hawk, the main characters spoke of what will now happen to the galaxy. Kreia stated that now that the Jedi are exterminated, the Sith will betray each other.
On Darth Nihilus's ship, the Ravager, Sion confronted Nihilus claiming that he was the more powerful now that the Jedi were no more. He drew his lightsaber but Nihilus was quicker, attacking him with the Force. Sion realized he was not so powerful after all and left in shame.
Some time after, T3-M4 reawoke the Exile, who was not dead after all. They returned to the Ebon Hawk and left M4-78.
Behind the scenes
M4-78 was originally created for Star Wars: Knights of the Old Republic II, but was cut from the game due to time constraints, although at least one mention of the planet can be found in the game. It has been acknowledged, however, that the Jedi Exile never visited the planet.[source?] The planet became canon when referenced in the short story MedStar: Intermezzo. Although displayed during a showing at E3, it was cut out of the final game for a number of reasons. One of the major reasons, however, is that because Obsidian was being pushed by LucasArts to complete the game by Christmas, they had to make some cuts to the game in order to finish development. M4-78 was one of those cuts.
Other reasons for the cut was because the other planets in the game didn't have as much content as was preferred, so a majority of the elements on M4-78 were moved to other planets in the game. For example, the shuttle-docking puzzle on Nar Shaddaa was originally a puzzle on M4-78. Another reason why it was cut was because its plot was nearly identical to the Peragus Mining Facility section of the game.
While very complete dialogs, and in some cases voice-overs, exist within the game's files for Vash, Kaah, IS-24, and ES-05, absolutely nothing remains or was ever implemented for M4-78 itself, severely hampering the ability of modders to fully understand its role in the plot.
M4-78 was divided up into 7 modules named: Landing Arm (where the Ebon Hawk landed), Central Zone (originally flooded with radiation, led to most other areas), Environment Control Zone (originally flooded with radiation, where Kaah first impersonated ES-05), Industry Control Zone, Archon I Behavior Core (where ES-05 was housed, where the PC met Kaah), Archon II Behavior Core (where IS-24 was housed) and Main Behavior Core (where M4-78 was housed, where the PC met Vash).
M4-78 has been restored as a mod. 
- Star Wars: Knights of the Old Republic II: The Sith Lords (First appearance) (Cut content)
- "MedStar: Intermezzo"—Star Wars Insider 83 (Mentioned only)