The Phantom Menace's "Duel of the Fates" is inarguably the most important lightsaber duel in the Skywalker saga, so much so that Darth Maul's victory would have changed the entire trajectory of the Star Wars galaxy's future. Although the Sith Lord apprentice to Palpatine slew Jedi Master Qui-Gon Jinn on the planet Naboo, Maul was defeated by Obi-Wan Kenobi and was believed to have been dead in the decade that followed before the dawn of the Clone Wars. However, one has to wonder what would have happened if Maul had slain both Jedi and emerged triumphant.
In The Phantom Menace, Obi-Wan and Qui-Gon's mission to protect Queen Amidala of the Naboo and end the Trade Federation blockade finds them on Tatooine where they meet young Anakin Skywalker. Sensing the boy's great power in the Force, Qui-Gon believes Anakin is the Chosen One and intends to train him in the ways of the Jedi despite the reluctance of the Council. However, the duel with Maul results in Qui-Gon's demise, resulting in Obi-Wan becoming Anakin's master after Kenobi avenged his own master's death. The fate of the galaxy was decided in the aftermath of this battle, though the future would have been a lot darker had Maul won.
