In Star Wars, the battle between the Light and Dark Sides of the Force is something that every Jedi Padawan must go through. They must experience the tug of both sides and decide for themselves which path they will follow. While the majority of will end up choosing the Light side of the Force, that internal struggle continues as they become Jedi Knights.
Throughout Star Wars, many of the greatest Jedi have eventually succumbed to the temptation of the Dark Side of the Force. Recognized as "Fallen Jedi," these powerful Force users have become some of the toughest villains the Jedi Order ever faced.
10 Beldorion
While the Hutt seen in Star Wars television series and films are all seen as villains, Beldorion is one of the few who starts as a hero. He served as a Jedi Knight for centuries until the fall of the Galactic Republic and the rise of the Galactic Empire. His fall came about after being sent to Nam Chorios.
Beldorion forsook the Jedi Order and took control of the planet, ruling with an iron fist. Easily corrupted, he would enslave many of his subjects. Eventually, Leia Organa battled and defeated him in lightsaber combat, making him the second Hutt she killed.
9 Darth Krayt
Darth Krayt began life as A'Sharad Hett, a Jedi Knight in the dying days of the Galactic Republic. After surviving Order 66, he returned home to lead his family of Tusken Raiders. He would eventually give in to the Dark Side of the Force following the defeat of Darth Sidious in Return of the Jedi.
Darth Krayt founded the "One Sith" movement. Rather than follow the "Rule of Two" enacted by the Sith for a millennium, the "One Sith" members were heavily organized and actively participated in bureaucracy. This movement led the Sith back to a place of power in the days after the fall of the Empire.
8 Dagan Gera
Dagan Gera was one of the most loyal Jedi Knights during the High Republic. Unfortunately, he became obsessed with expanding the Jedi Order's objectives in the Outer Rim, specifically the hidden planet to Tanalorr. When the Jedi Council ordered him to abandon the planet, Dagan betrayed the Jedi and was placed in a bacta tank after being injured.
When he was freed from the tank, Dagan learned the Jedi Order had fallen. He then committed himself to the Dark Side of the Force. He became strong enough to control his lightsaber with just the Force after losing his arm in battle. His rage at the fall of the Jedi Order fueled his Dark Side powers.
7 Darth Caedus
One of the more tragic falls from grace in the Expanded Universe, Darth Caedus was born Jacen Solo, the son of Han Solo and Leia Organa. Jacen was a champion of the Galactic Alliance before succumbing to the Dark Side of the Force. His desire to protect the galaxy became his undoing as he was willing to accept any cost to get what he saw as protection.
After becoming Darth Caedus, he went on a reign of terror to continue his efforts to bring what he thought was peace and stability to the galaxy. Simply put, Darth Caedus followed the same path his grandfather did to become Darth Vader.
6 Kylo Ren
Ben Solo's story is almost as sad as Jacen Solo's story. In the films, Ben is the son of Han and Leia, but he flees Luke's Jedi Academy after Luke almost kills him. This begins Ben's descent into villainy. While not a full Jedi Knight like Jacen was in the Expanded Universe, Ben still went on to become a dark figure in the sequel trilogy as Kylo Ren.
There are a few differences between Ben and Jacen. While Jacen fell because of similar motivations to Anakin, Ben fell and then modeled himself after Darth Vader. Ben would kill his father and nearly kill his mother before ultimately finding redemption.
5 Asajj Ventress
While she was never officially a member of the Jedi Order, Asajj Ventress was trained in the ways of the Jedi by Ky Narec. Ventress was an orphaned Nightsister who eventually fell to the Dark Side of the Force and worked with Darth Sidious and Count Dooku in an effort to defeat the Clone armies of the Galactic Republic.
Ventress was one of the most powerful Dark Side Force users during the Clone Wars. She survived the massacres of the Nightsisters and even survived after Count Dooku left her for dead. She would eventually fall in love and move from Fallen Jedi to Gray Jedi, later giving up her own life to save Quinlan Vos.
4 Barriss Offee
Training under Jedi Master Luminara Unduli, Barriss Offee was a skilled Padawan who fought in the Battle of Geonosis and the Clone Wars. She was skilled both as a warrior and a diplomat, which made her a dangerous foe once she lost faith in the Jedi. Barriss eventually started viewing the Jedi as warmongers as opposed to peacekeepers, leading her to leave the Jedi Order and renounce the Jedi Council. While her views may have been legitimate, her methods were not.
Barriss bombed the Jedi Temple and framed Ahsoka Tano for the crime. Her actions resulted in the murders of Jedi, crew, and clones. It also led to Ahsoka's own departure from the Jedi Order, even though Ahsoka never gave in to the Dark Side. Barriss was eventually defeated by Ahsoka's Master, Anakin Skywalker, who cleared his Padawan's name.
3 Baylan Skoll
Baylan Skoll is one of the many Jedi who survived Order 66 only to fall to the Dark Side of the Force. He worked with Morgan Elsbeth in an attempt to bring back Grand Admiral Thrawn after the fall of the Empire. Baylan Skoll knew Anakin Skywalker before his own fall, which allows him to exploit Anakin's relationship with Ahsoka when the pair are fighting.
Baylan seems to truly respect the Jedi Order, demonstrating sadness any time he has to strike down any other surviving Jedi. However, he is still able to take down many of them with ease, thanks to his prowess with his lightsaber. Baylan Skoll even takes on his own Dark apprentice after his fall.
2 Count Dooku
Count Dooku is the most unorthodox Jedi in this history of the Jedi Order. Descending from royalty, Count Dooku trained to become a Jedi and even served as a Master to Qui-Gon Jinn when he was a Padawan. Like many other Jedi, Dooku lost faith in the Republic and left the Order to return to his home world.
After leaving the Order, he began training under Darth Sidious, becoming the Sith apprentice Darth Tyrannus. He was a major leader in the Separatist movement leading up to and during the Clone Wars. Dooku would cut off Anakin Skywalker's hand before eventually dying by Anakin's blade. He was both ruthless and powerful.
1 Darth Vader
No fallen Jedi is more recognizable as Darth Vader. Born as a slave on Tatooine, Anakin Skywalker joined the Jedi Order after being freed by Qui-Gon Jinn. Under the training of Obi-Wan Kenobi, Skywalker would grow strong in the Force becoming one of the greatest Jedi Knights of all time. But Skywalker would get married against the wishes of the Council, and his desire to protect his love led to his downfall.
While trying to prevent Padme Amidala from dying, Anakin turned to Darth Sidious for help, leading to him betraying the Jedi Order and becoming Darth Vader. He helped usher in the Galactic Empire, killed younglings, and nearly killed his own children while at Sidious' side.