The Master Saber is an extremely powerful lightsaber. It was created by the dark side of the Force.
During the Rise of the Empire, the Master Saber was hidden in a place where all of the dead Sith warriors and Lords go to after they have died. In 32 ABY, Darth Raiden visited that place, witnessed the power, and killed an unknown undead Sith Lord that wielded it. After taking the almighty lightsaber, he combined the light and dark side into the saber so that it will be more powerful than anyone can possibly imagine. Raiden then went to the center of the galaxy and killed Darth Metallix, the God of the Sith, using both the Master Saber and the god's weapon. Metallix's minions bowed before Raiden, and he became the new God of the Sith.
Using the Master Saber, Raiden can kill any opponent, even a Jedi, with one strike. When a Jedi gets slashed by the Saber, all of his/her Force energy gets drained into the Saber, so that he/she will never, ever become a Jedi spirit. Also, if a Jedi or mortal gets struck by the Master Saber, he/she will never ever come back to life. The only thing that the Master Saber can't kill is its master, Darth Raiden.
- Aside from the other weapons used by the Sith, the Master Saber so far is the deadliest and most powerful weapon the Sith have ever created. As for Darth Raiden, it is the deadliest and most powerful weapon he has ever wielded.