The rise and fall of Anakin Skywalker is the foundation of the entire Star Wars franchise. Anakin's turn to the dark side is what many Jedi Masters fear their apprentices might do. Plenty of films, books and comics emphasize light side users struggling with this temptation. But there were also Force users on the dark side who changed their ways as well. Many Sith throughout Star Wars canon and legends were redeemed and turned to the light.
Flint (Legends)
Flint was a Sith Lord who trained under Darth Vader, whom he considered to be his surrogate father. Flint and Luke may have shared the same "father," but they also had similar backstories. Both of Flint's real parents died. His father, Jedi Knight Flint Torul, was killed during the Clone Wars by General Grievous, but young Flint never met him. Still, he was entranced by the idea of becoming a Jedi like his dad. When Luke Skywalker visited his home planet, he was determined to become his apprentice. But Luke rejected him, and this devastation, along with the heartbreak of losing his mother shortly thereafter, caused Flint to become bitter. In his state of despair, Vader found him. The Sith Lord sensed Flint's innate power and offered to train him. Thus began Flint's descent to the dark side.
After years of Sith training, Flint turned to the light. He got a chance to take revenge on Luke, as the two engaged in a vicious lightsaber duel that ended in a stalemate. It was after this fight that Flint's presumed-dead friend showed up and convinced him to turn from his wicked ways. This was enough to make Flint reject the dark side, and he lived out the rest of his days on his home planet until he was killed by Lumiya, one of his previous Sith Masters.
Darth Sajar (Legends)
A Dark Lord of the Sith and a member of the Dark Council, Darth Sajar was very powerful in the Force. He eventually became a Sith Lord during the reign of the Sith Empire, but before long, he came in contact with a Jedi Master who helped him turn to the light.
This Jedi was Master Tol Braga, a sage pacifist who desired peace. He had contacted many Sith during the Great Galactic War in an attempt to reconcile without further violence. Sajar took advantage of this and lured Braga to the planet Dantooine, where he intended to kill him. The Sith Lord and the Jedi Master dueled for three days straight while debating both sides of the Force, offering arguments for the dark and light, respectively. The battle finally ended when Braga convinced Sajar to convert to the light side and become his Padawan.
Kel'eth Ur (Legends)
Another powerful dark side user during the Sith Empire was Kel'eth Ur. He studied as a Sith until he adopted the Jedi's philosophy of the pursuit of peace, realizing that Sith were ruled by fear. Ur then turned from the dark side and learned the ways of the light, imparting his knowledge to others. But the Sith didn't let him off so easily, and the Emperor eventually killed him for heresy.
Darth Gravid (Legends)
Even during his time as a Sith Lord, Darth Gravid faced internal conflict in fully committing to the dark side. Convinced that total devotion to the ways of the Sith would be his undoing, he attempted to integrate aspects of the light side -- altruism and empathy -- into his practice.
Though conflicted by both sides of the Force, Gravid was still devoted to destroying the Sith. He believed the only way to preserve their empire was to destroy all Sith artifacts, teachings and holocrons. Gravid thought that once this was done, he would be able to convince his peers to change their ways. He began destroying various artifacts until his former apprentice, Darth Gean, arrived to stop him. They engaged in a battle in which the former Sith was murdered.
Darth Revan (Canon & Legends)
Darth Revan is arguably one of the most powerful Jedi/Sith Lords in the continuity and a character favored by many Star Wars fans. Although he was brought into the main canon through a brief mention in Pablo Hidalgo's Star Wars: The Rise of Skywalker: The Visual Dictionary, Revan's backstory is established in Legends.
Revan had an arc similar to Anakin Skywalker's, being that he followed the light side, then the dark, then the light once more. He was found by the Jedi Order in the Outer Rim territories and became Jedi Master Kreia's Padawan. Revan displayed an insatiable hunger for knowledge and soon became the Jedi's most talented student. His power only grew as he aged, and he led many organizations and armies into battle. One such battle was the Mandalorian Wars, during which he became disenfranchised with the Jedi Order. Adopting the dark side, he obliterated the leader of Mandalore, ending the war.
Along with his once-Jedi friend, Darth Malak, Darth Revan retreated to the Outer Rim where he met Sith Lord Vitiate. Both fell under Vitiate's mind control and became his apprentices. Eventually, the pair broke free and created their own powerful Sith Empire, waging war on the Jedi and the Republic in the Jedi Civil War. Revan almost succeeded, but Malak turned on him, nearly killing him.
The Jedi saved him and took him back to the Council, wiped his mind, and retrained him as a Jedi to take down the Sith. Revan eventually attempted to assassinate the Emperor, but failed and was killed. However, his skill and power in the Force granted him the ability to wage telepathic war on the Sith for three hundred years.
Darth Vader / Anakin Skywalker (Canon & Legends)
Dubbed the Chosen One due to his immense innate power in the Force, Anakin Skywalker trained as a Jedi under Obi-Wan Kenobi. He fought in the Clone Wars and showed great promise as a student of the Order. However, he became disenfranchised with the Jedi, and soon fell under the influence of Emperor Palpatine. The Sith Lord gave him the name Darth Vader and sent him to massacre the Jedi, leading to the reign of the Sith Empire over the galaxy.
However, the light side of the Force was still within Vader. He served as a devoted Sith Lord for many years, until his son, Luke Skywalker, confronted him in Return of the Jedi. It was then that he denounced the dark side and lived on as a Force ghost.