This lightsaber, distinguished by its red blade, was the Sith weapon employed by Starkiller, the code name for Galen Marek, who served as the secret apprentice to the Dark Lord Darth Vader. Marek was a Force-sensitive assassin. The design of the apprentice's lightsaber was stripped down, appearing skeletal and purely functional. It was created to serve in both training and combat, fulfilling roles such as the many sparring matches between the apprentice and his holodroid PROXY, and meditation where he would use telekinesis to re-assemble it for modifications.
As his training approached its conclusion around 3 BBY, the apprentice received orders to eliminate three fugitive Jedi Masters: Rahm Kota, Kazdan Paratus and Shaak Ti. After he completed his final trials, Vader betrayed his apprentice under the instructions of Emperor Palpatine, and ejected him into the emptiness of space. Consequently, Marek's lightsaber was lost to the vacuum, though his Sith Master ultimately saved him from death so that he could be redeployed on new missions that would ultimately lead to the formation of the Rebel Alliance. Because his apprentice lost his lightsaber, Darth Vader gave Marek another lightsaber as a replacement.
The first lightsaber of Galen Marek seemed to take inspiration from the design that Darth Vader himself used, including a largely solid casing and an emitter shroud. However, Marek chose a much more skeletal construction, rather than using the bulky design of his master's weapon. His weapon had an exposed crystal chamber, and the entire weapon was covered in what appeared to be tarnished bare metal, instead of a handgrip. The blade utilized a set of red crystals provided by Vader.
The circumstances surrounding the creation of the lightsaber, like much of Galen Marek's training, were quite harsh. Marek once commanded PROXY, when he was a child, to chain him up and deny him food or water until he could disassemble and reassemble the lightsaber using only the Force. He was unsuccessful, and PROXY had to release him. While this initial attempt failed, Marek eventually succeeded in building a lightsaber, which he used frequently in training duels with his droid, PROXY, and even with Vader. One such fight lasted for hours, with Vader simply defending against Marek until the apprentice collapsed from exhaustion. Marek was once sent on a mission to Yuzzem homeworld Ragna III, where he discovered that Vader had intentionally broken the lightsaber, forcing Marek to use his Force powers to fight off the Yuzzem.
Marek first used the lightsaber against a Jedi when he was tasked with eliminating Rahm Kota, who was attacking an Imperial shipyard above Nar Shaddaa. He fought his way through the facility, killing Imperials and Kota's men alike, in accordance with Vader's instructions to eliminate all witnesses. Marek finally confronted Kota in the shipyard's command center. After a fierce battle, Marek defeated Kota by blinding him with his own blade, thrusting it into his face during a blade lock. The lightsaber was later used against Kazdan Paratus on Raxus Prime, however Marek was compelled to defeat Paratus using his Force abilities, due to Paratus' proficiency with a lightsaber.
While meditating on his way back to Vader, Marek used telekinesis to disassemble and reassemble his lightsaber. However, a communication from Vader interrupted him, breaking his concentration and scattering the lightsaber parts across the floor of his ship's training room. Marek was ordered to travel to Felucia and eliminate Shaak Ti, one of the last surviving members of the Jedi Council. Marek barely managed to defeat Ti, impaling her with his own blade while barely blocking her attack with telekinesis. Upon returning to Vader to report his success, Marek was betrayed because Emperor Palpatine had discovered his existence. Marek was thrown into the vacuum of space on Palpatine's orders, and the lightsaber was lost to the void.
Galen Marek's almost dead body was quickly recovered, and after he recovered, he continued to serve Vader, using Kota's lightsaber to replace his original weapon.
The disappearance of Galen Marek's lightsaber is a plot point that is only present in the Star Wars: The Force Unleashed novelization, as the various game versions and the graphic novel adaptation show Marek continuing to use his original lightsaber, albeit with a blue blade. Similarly, the game versions allowed Marek to swap out the hilt and crystals with others that the player could collect. However, because of the inconsistencies between the different game versions of the story on several plot points, the novelization is generally considered to be definitive, so the loss of Marek's lightsaber is considered part of Legends canon.
In the non-canon dark side alternate ending, Marek defeats Darth Vader, but is defeated by Darth Sidious on the Death Star and rebuilt as a cyborg to become his apprentice. In the Ultimate Sith Edition DLC, Lord Starkiller can be seen with his old lightsaber on his belt, along with the lightsabers of Darth Vader, Rahm Kota, Darth Maul, and the top section of Kazdan Paratus's lightsaber pike. The Sith Lord has started using a new lightsaber by this time.