In the entire galaxy, there's only one group of warriors who are strong enough to save it: the Jedi Order.
The Jedi Order has been around for thousands of years, doing everything in their power to make sure that the galaxy remains safe. Whether it be participating in brutal wars or serving as personal bodyguards to at-risk folks, the Jedi are willing to do whatever it takes to bring balance to the galaxy and to the Force.
That doesn't mean that they are without fault, though. Numerous times in Star Wars history have the Jedi actually done wrong in favor of their own pride and accomplishments. They may want to save the galaxy, but they often can't get past their own strength and wisdom as a result. As they would come to learn, these mistakes would be grave.
Due to the morally ambivalent nature of the Jedi, many young Padawans and Knights fall from grace to become powerful Sith Lords. Whether they were seduced or disillusioned with the state of the Order, they turned to the Dark Side in the end. Some of these Jedi ended up becoming the most powerful and feared Sith in the entire galaxy, leading to painful moments of heartbreak and tragedy.
This is not a happy story: rather a highlight of 15 fallen Jedi who turned into terrible Sith Lords. They instilled fear and sorrow into everyone they came in contact with, and most weren't safe from their wrath.
15 Ben Solo- Kylo Ren
Decades after the events of Return of the Jedi, Luke Skywalker took it upon himself to re-establish the Jedi Order. He began a batch of young Force-sensitive children. He had his own Jedi academy and trained them in the ways of the Light Side. All seemed well, but one of his padawans seemed different.
Ben Solo was the son of Han Solo and Princess Leia. He was Force sensitive and was trained by Luke, but he had more than one influence in his life. Supreme Leader Snoke began pulling the strings and seduced Ben to the Dark Side, changing him into the emotionally unstable leader of the Knights of Ren, Kylo Ren. Kylo Ren then became a prominent member of the First Order and utilized advanced Force techniques in order to gain information or kill prisoners. He took heavy inspiration from his grandfather, Darth Vader, and sought to be a much more powerful Sith Lord and even went so far as to kill his own father. As it stands right now, Ren is finishing his training under Snoke, likely to officially become a Dark Lord of the Sith. And what a transition that will be.
14 Quinlan Vos
Quinlan Vos was another unique Jedi. He was well-known for being an excellent fighter, but also had the tendency to go against the Order and do what he wanted. However, everything he did was for the good of the galaxy and he simply ignored all of the politics involved. His specialty was going on undercover missions to discover all kinds of intel against the Sith. One mission changed his life forever.
Vos was sent on a secret job by the Order to kill Count Dooku (which they neglected to tell him about in the beginning), and along the way teamed up with Asajj Ventress. During the mission, he was captured and tainted by Count Dooku. Becoming a Sith himself, it seemed there was no more hope for Vos. Yet even in darkness, Vos still despised the Sith and planned on using his new powers to overthrow the Emperor. However, Ventress eventually returned and their love single-handedly brought him back to the Light Side (it was not some sort of mind trick by the Order, interestingly enough). He was then classified along the likes of Darth Revan for having been a Sith then returning to the Jedi Order.
13 Exar Kun
"I was the greatest Dark Lord of the Sith."
When a quote like that comes from a fallen Jedi, you know they mean business. Exar Kun was a powerful and brilliant Jedi Knight, but he was always motivated by the desire to do more. He always had a natural curiosity for the ways of the Sith and decided to pursue that knowledge despite that it was forbidden. He then decided to embark on a journey to discover everything he could about his enemies, which eventually brought him to the Sith planet of Korriban. It was there that he was tricked and tortured by a Sith ghost, and he became one of the greatest Dark Lords of the Sith.
Kun's leadership brought back a golden age for the Sith Empire. He trained more apprentices and prepared everyone for battle. With his army in hand, Exar Kun brought war straight to the Galactic Republic as well as the Jedi Order. Many people died at his hands and the times seemed darker than they had ever been. He didn't even die when he lost the war: Kun performed some dark magic that expelled his spirit from his body so that he could live on to persuade other Jedi to defect.
12 A Sharad Hett- Darth Krayt
A Sharad Hett makes it on a lot of lists like this one and that's because his story is so fascinating. He began life as a Tusken Raider, but when a Jedi noticed his potential, he was brought into the Order. He was trained very well and was prepared for when all Hell broke loose during Order 66. Hett's mentor sacrificed himself so that the apprentice could escape. After that, Hett went back to Tatooine to lead the Tusken Raiders. He expanded their territory, but his rule became so fierce that it caught the attention of Obi Wan Kenobi. Hett lost his arm and his position after that duel.
From then on, Hett decided that he no longer wanted to be a Jedi and became a Dark Lord of the Sith named Darth Krayt (in honor of a powerful beast that he took down). Darth Krayt began to rebuild the Sith Empire years after the Galactic Empire crumbled to the ground. He trained many other Sith Lords and brought a terrifying rule to the galaxy. He also became a primary antagonist for Cade Skywalker, a descendant of Luke.
11 Barriss Offee
Many times it is the young Jedi rather than the old ones who became persuaded by the allure of the Dark Side. Barriss Offee was just another victim of it. She was trained by Luminara Unduli during the Clone Wars (a Jedi who was later captured and killed). Barriss was fairly confident in the ways of the Force and was willing to fight alongside the Republic and the clone troopers in those times.
Unfortunately, the Clone Wars weren't the best times for the Jedi Order (the Council in particular). The higher-ups made many decisions that people questioned, and this led many Jedi defecting to the Dark Side. Barriss was very hesitant to trust the Order and the Council, feeling that they had gone far from the ways that established the Jedi in the first place. Because of this, Barriss decided to take matters into her own hands and bomb the Jedi Temple. She was so dedicated to her cause that she allowed her friend, Ahsoka Tano, to be framed for the bombing as well. By the time Barriss was convicted, they were too late, and she had already become a patron of the Dark Side.
10 Phanius- Darth Ruin
It's one thing when a Jedi Padawan defects to the Dark Side, but another thing entirely when a Jedi Master does the same. Phanius was one of the greatest Jedi in the Order, but there was something quite different about him. He was known to spout very unique philosophical teachings that were questioned by many other Jedi. Because some of these topics were forbidden, Phanius was highly encouraged to not pursue them. Nonetheless, he hungered for more knowledge and wisdom and left the Order to satisfy that desire. Along the road, he turned to the Dark Side and became a Sith, but it doesn't end there.
After taking the name Darth Ruin, Phanius decided to rebuild a new Sith Order. He went all across the galaxy and gathered as many new Sith apprentices as he could from all kinds of clans. Together, he and the new Force Sensitive folk rebuilt the Sith Empire with himself at the head of it all. He waged war on the Jedi, beginning a brutal conflict. Unfortunately, his selfish desires led to some of the other Sith despising him, and he was later betrayed.
9 Tel Angor- The Force Vampire
You'll see a common theme among many of these entries: it's a result of the pride of the Jedi Order. Tel Angor was an excellent Jedi Knight who was heavily connected to the Force. While he was still on the Light Side, the Jedi Order decided to conduct an experiment that could turn a Jedi into a living beacon of everything good in the universe. Once they began testing, Tel Angor was the one who volunteered. He prepped for about a month, solidifying his connection to the Force, hoping to become the brightest warrior who ever lived.
Like most experiments, though, the test backfired. Instead of turning Angor into a better Jedi, it allowed the Dark Side to come in and corrupt him. Adding that with his connection to the Force, he became reliant on it so much that he had to feed off of it from other Force users. He became a dark and nasty creature that the Jedi feared, and the valiant Tel Angor became one of the scariest fallen Jedi to date. He raided all kinds of Jedi buildings and wiped them clean of every ounce of the Force. I'd suggest you sleep with your night light on this evening.
8 Darth Desolous
Another Jedi Master to be seduced by darkness, Darth Desolous began his life as an unnamed Jedi. He rose through the ranks until he became a powerful Jedi Master. Yet, he was not satisfied with all of the knowledge that he gained. He simply wanted more. He began studying Sith teachings, and was gradually corrupted as a result. Due to his indulging of dark teachings, he was expelled from the Order, and this sent him on a warpath.
He took the name of Darth Desolous and became a Dark Lord of the Sith. He then raised his own army of beings from his home planet. After training them how to fight, he led them in a series of battles and ambushes against the Jedi. He and his comrades were so skilled that around 2,000 Jedi Knights were killed. Darth Desolous revelled in these victories and became increasingly confident each time. As with most Sith, his pride became his ultimate undoing. He and his army were demolished by the Jedi. However, they still vividly remembered the name of Darth Desolous and his likeness would end up being used in the Jedi Trials.
When you watch The Phantom Menace, it's clear that the master Qui Gon Jinn has seen a lot of terrible things in his life, considering how he tends to make decisions that the Jedi Council often disagrees with. One event that shaped him was a padawan he trained several years before the movie took place. This young Jedi's name was Xanatos, and he was very promising, having both fighting prowess and superior intelligence. This did end up giving him some pride issues, but Qui Gon was willing to look past that in order to train the young boy.
As Xanatos improved in his abilities, his final test was to go to his home planet with Qui Gon to end a crisis that was started by his own father. Once Xanatos got there, he began to be seduced by the desires of his father. Then after witnessing his sister die, Xanatos failed the final test and became a Dark Jedi. Due to his expertise in architecture, Xanatos then drew up plans to bring down the Jedi Temples. However, Qui Gon was ready to face his old apprentice with the young Obi Wan Kenobi at his side. Interesting how things happen.
6 Darth Revan
During the days of the Old Republic, there was a powerful Jedi Knight by the name of Revan. In those times, the Sith were increasing in size and power, and Revan was sent to investigate with a friend. Sadly, Revan and his friend were both captured and tortured by the Sith. Over time, the corruption entered their minds and Revan was turned into a Dark Lord of the Sith: Darth Revan.
As Darth Revan began operating and killing those who opposed him, the Jedi thought that it was high time for an intervention. They ended up capturing Darth Revan and erasing his memory of his time as a Sith. When he awoke, all he remembered was that he was a Jedi. He then came back to the Light Side and earned the name of the Prodigal Knight (one of the only Jedi to ever receive this title). Unfortunately, there were still some loose ends that needed tying because of his reign as a Sith Lord. Deciding to fight for what was right and good in the universe, Revan took up his blue lightsaber once more in an attempt to bring peace. This led to a brutal confrontation with his old friend.
5 Alek- Darth Malak
There is a bit more to the story of Darth Revan, and it includes his friend Alek. Alek was a Jedi Knight alongside Revan, and they had a history of going against what the Jedi Order commanded. After deciding to go on a mission to the Outer Rim, the Order issued an arrest warrant for Alek and all the Jedi that went with him. He changed his name to Malak to avoid being incarcerated, and worked with Revan to find as much as he could about the current Sith.
As you know, this led to their capture and torture. Like his comrade, Alek was also corrupted into a Sith Lord. He took the name Darth Malak and served as the apprentice to Darth Revan (complete with a cool mask and lightsaber). However, as the war exploded and Darth Revan went back to the Light Side, Darth Malak was forced to step up to the plate. He trained other Sith apprentices and fought in a powerful duel with the redeemed Revan. Unfortunately for Darth Malak, it ended up costing him his life. Not only did this duel mark the end of Darth Malak, but the Jedi Civil War as well.
4 Asajj Ventress
Understand that not all of the fallen Jedi were a result of the Jedi Order directly. Sometimes people go to extremes in order to do what they feel is necessary. Asajj Ventress lived her life as an orphan before being discovered by the Jedi Narec. She was then brought into the Order and trained in the ways of the Force. She was very proficient with a lightsaber and was very promising overall. She and her master protected the planet Rattatak for about a decade.
However, some pirates came to town and killed Narec, which sent Ventress on a path of vengeance. In order to kill those that had orphaned her once more, Ventress opened her heart to the power of the Dark Side. She killed the evil lords of the planet and took their place. It was at this time that she was met by Count Dooku, who trained her as his Sith apprentice. It wasn't long, though, before the Emperor feared Ventress's power and asked Dooku to kill her. Ventress did manage to escape and became a bounty hunter with the mission to take down Count Dooku. Little did she know that this desire would be her undoing.
3 The Grand Inquisitor
Another Jedi to have fallen around the time of the Clone Wars, the Grand Inquisitor was once a high-ranking member of the Jedi Order. He was one of the Knights present when the Order was attempting to convict who had bombed the Jedi Temple. However, instead of doing further investigation, they decided that it was Anakin Skywalker's padawan, Ahsoka Tano, who was guilty. Then it came out later that Barriss Offee was the real culprit. This lack of understanding displayed by the Order caused the Knight to begin hating the Order. He defected and became an Inquisitor for the Dark Side.
Rising through the ranks, he ended up becoming the Grand Inquisitor: one of the deadliest Jedi hunters there ever was. The Grand Inquisitor fought numerous hordes of Jedi and hunted down any stragglers that were left after Order 66 came to a close. He would later be present in the Star Wars Rebels series and try to seduce Ezra Bridger to the Dark Side. Eventually he would lose in battle and rather than go back to his master, he allowed himself to die, fearing that the consequences for his failure would be far worse.
2 Count Dooku
Dooku was brought from his childhood into the Jedi Order. He had the great honor of being trained by Master Yoda. As he was taught in the ways of the force, it wasn't long before he rose to the rank of Jedi Master. As he held this title, he received and trained the padawan Qui Gon Jinn. After Jinn became a master of his own, Dooku started to notice the politics within the Jedi Order and was angered as a result. Once he got the news of his apprentice dying, Dooku left the Light Side and went back to his homeworld to rule as its count: henceforth the name Count Dooku.
Then the Emperor came along and recruited Dooku to the Dark Side and renamed him Darth Tyranus (though he still chose to be addressed as Count Dooku). Dooku then became one of the driving forces of the clone wars, supplying the clone template as well as constructing powerful droids to use in the armies. He was also responsible for training General Grievous, easily one of the greatest Jedi hunters in existence. However, Dooku's partnership with the Emperor led to his demise in Revenge of the Sith.
1 Anakin Skywalker- Darth Vader
Perhaps the greatest of all fallen Jedi stories, Anakin Skywalker is a sad tale from beginning to end. From the very beginning, his existence was orchestrated by Darth Plagueis and Darth Sidious (Plagueis was the one who created him). Then Qui Gon took it as a sign of him being part of the prophecy. Despite the hesitation of the Council, Anakin was taken on as an apprentice, but by Obi Wan.
Because Obi Wan wasn't adequately prepared to train the young Skywalker, Anakin grew proud and dangerous. He immediately saw the faults within the Order and questioned their every move. It wasn't long before one thing led to another, and he was turned into one of the greatest Dark Lords of the Sith who ever lived: Darth Vader. Vader had Force powers unlike any other Force user at the time. After Order 66 was put into effect, he also became a Jedi hunter to kill off any stragglers. Darth Vader was the most game-changing Sith to ever be created, and his betrayal signified a change in the galaxy forever. Still, he managed to realize his wrongdoings and bring balance to the Force in the last days of his life.
Sources: Starwars.com, Wikia