The Sith, the main villains of the Star Wars universe across it's various incarnations, have taken various forms over their existence, from the mighty but discord-filled Sith Empires of old to the shadowy "Rule of Two" practitioners from Darth Bane's lineage.
While most famously represented by figures such as Darths Vader, Sidious, and Maul, the now discarded Legends expanded universe filled in the gaps of the Sith's history by introducing several Dark Lords who had lived throughout the history of the galaxy far, far away. Here are some who deserve another shot at being a part of the Star Wars canon.
10 Darth Zannah
Apprentice to Rule of Two founder Darth Bane, Zannah was once an innocent orphan named Rain until her friend Laa was mistakenly killed by a Jedi. Lashing out and murdering the Jedi in turn with the Dark Side, Rain's outburst attracted the attention of Bane, who molded the young girl into his apprentice. Made cruel and powerful in equal measure by her training, Zannah ended up being the first to fulfill one of the central tenets of her master's philosophy when she murdered Bane in a duel on planet Ambria then usurped his place as Master of the Sith. Darth Bane himself has been established in the current Star Wars canon, fulfilling the same role he did in Legends as founder of the Rule Of Two, so it's only natural that his apprentice should follow suit.
9 Darth Cognus
Were Darth Zannah to be added back to canon, the natural next step would be to re-canonize her own apprentice, Darth Cognus. Originally an Iktochi (the same species as Jedi Master Saesee Tiin) assassin known only as The Huntress, the future Cognus was recruited by Darth Bane as an alternative apprentice, when he felt Zannah was simply waiting for him to die naturally rather than challenge him for mastership outright. However, before Cognus' training could begin, the challenge Bane craved from his first apprentice came. Pledging herself to the victor of the duel, Cognus fell under the apprenticeship of Darth Zannah. A skilled warrior and clairvoyant to boot, Darth Cognus is a figure who deserves to have more tales told of her storied and quite violent life.
8 Emperor Vitiate
Born as Tenebrae and later known as Valkorion, Lord Vitiate presided over a reign of chaos and destruction in the galaxy for over a thousand years, until he was finally defeated around 3500 years before the Battle of Yavin. The bastard son of Sith Lord Dramath, Vitiate ascended in the ranks of the Sith to rule planet Medriaas.
Following the Sith's defeat in the Great Hyperspace War, Vitiate lured the survivors to his domain, where he drained their life-force and that of the entire planet, becoming immortal in the process. The only Sith Lord in history to unlock the secret to true eternal life, Vitiate's actions were felt across galactic history. Such a storied figure deserves at least a nod in the new canon, especially considering his role in the lives of other Sith on this list...
7 Darth Revan
The star of the hugely popular Knights Of The Old Republic, Revan was born almost 4000 years before the events of the original Star Wars trilogy. Once a Jedi and hero of the Mandalorian wars, he was corrupted by Sith Emperor Vitiate and began his own Sith Empire until being defeated by Jedi Bastila Shun. When his memory erased by the Jedi Council, Revan returned to the Light, even after rediscovering his identity, and began a crusade to undo the evil he'd done as a Sith. However, a holocron left by Revan inspired Darth Bane in his creation of the Rule of Two, so even though he died on the Light, his actions as a Sith led to the Dark Order's ultimate victory thousands of years later. Revan appeared alongside Bane in a deleted scene from The Clone Wars episode Ghosts Of Mortis, and received a name-drop in The Rise Of Skywalker visual dictionary, so there's clear amenability inside LucasFilm towards reincorporating Revan into canon. Let's hope they follow through.
6 Darth Malak
If Revan were to return to Star Wars canon, then his apprentice and eventual successor would be a natural fit to bring back as well. Similar to Revan, Malak was once a Jedi who fell under the sway of Vitiate, and became the right-hand of Revan's Sith Empire. Resentful of his master's authority, Malak lost his lower jaw in a lightsaber duel with Revan, and once his master disappeared, Malak was all to happy to write him off as dead. However, Malak's reign would eventually end on the Lightsaber of a returned Revan, now restored to the Light.
5 Darth Traya
Another Old Republic era Sith, Darth Traya debuted in Knights Of The Old Republic II, initially as a Jedi sage named Kreia who comes to mentor the protagonist. Her true identity and colors are eventually revealed however, as is her plan to destroy The Force altogether so that the Galaxy may be free of its predestination. Her plan is foiled by KOTOR II protagonist The Exile, however, and Traya perishes on Malachor V.
The planet of her death was revisited in Star Wars Rebels, where a Sith Temple served as the site of Darth Vader and Ahsoka Tano's clash; housed in the Temple was a Holocron, with a female voice reminiscent of Traya's. Furthermore, Traya's view that the endless conflict between Light and Dark is a futile war laid thematic groundwork for Luke Skywalker's similar questioning in The Last Jedi. Let's hope LucasFilm bites the bullet sooner rather than later and brings Darth Traya into their new canon outright.
4 Darth Nihilus
The unknown is often the scariest thing of all, and no Sith Lord proves that more than Darth Nihilus. The final Old Republic Sith on this list, Nihilus was exposed to a Super Weapon known as a "Mass Shadow Generator" on Malachor V. Transformed into a vampire of sorts, Nihilus began to crave the energy of the Force that sustains life, and eventually lost his physical form altogether, becoming a Dark Side Wraith encased in frightening armor and a mask. The ultimate personification of losing one's self to the Dark Side, Nihilus deserves another chance to terrify Star Wars fans in the new canon.
3 Darth Krayt
Born to Jedi Sharad Hett on Tatooine, the future Darth Krayt (A'Sharad Hett) was once the Jedi Padawan of Ki-Adi-Mundi and a friend of Anakin Skywalker. Following Order 66, Hett returned to his home planet. Through an encounter with Obi-Wan Kenobi and learning that his former friend had become Darth Vader, Hett exiled himself and became a bounty hunter. Ironically enough, Hett eventually fell to the Dark Side himself; naming himself after the Krayt Dragon of his homeworld. Darth Krayt became ruler of the One Sith, the villains of Star Wars: Legacy. Essentially two characters in one, Darth Krayt would be an exciting (re)addition to the Star Wars canon, whether in his Jedi persona or his Sith one.
2 Darth Tenebrous
Darth Sidious was trained by Darth Plagueis, but then who trained Plagueis? Legends has an answer. A Bith name Rugess Nome and a Starship designer in his public life, Tenebrous manipulated the parents of Plagueis into meeting one another then took their offspring as his apprentice, though such apprentice would eventually be the death of him. Name-dropped in The Rise Of Skywalker visual dictionary alongside Revan, Tenebrous is a natural inclusion if LucasFilm ever wishes to revisit Darth Plagueis. Which brings us to our final entry
1 Darth Plagueis (Hego Damask)
This may seem an odd choice; Plagueis is the only character on this list mentioned in the films themselves, and it's still canon that he was the necromantic Sith who trained Palpatine. However, these scant facts are all we know about him. The character of Plagueis as he was established in Legends, including his species (Muun) and birth name (Hego Damask), have yet to be revealed in current canon. Given how masterful James Luceno's eponymous 2012 novel about Plagueis was, it seems a waste to let these details fall by the way side.