Much of the Star Wars franchise centers on the battle between good and evil, as typically represented by the Jedi and Sith, respectively. However, just as there are non-Jedi who can tap into the Light Side of the Force, not everyone who draws their power from the Dark Side is a Sith Lord. In fact, there are several other factions across the Star Wars galaxy that use the Dark Side, including some that aren't necessarily clear-cut villains the way the Sith are.
That said, here's a rundown of all the Dark Side groups in Star Wars that are not the Sith. This list will only include factions from the Disney canon, rather than the old Expanded Universe, aka Star Wars Legends.
Acolytes of the Beyond
A group of Dark Side fanatics, the Acolytes of the Beyond were non-Force sensitive beings who believed all living things must bend to the Force's will. However, they saw Sith Lords as the only ones capable of using the Force for their own purposes and worshiped Sith who had died. The Acolytes were active during the Galactic Civil War and sought to return Darth Vader's lightsaber to him after his death, as detailed in Chuck Wendig's Star Wars: Aftermath novel trilogy. In The Rise of Skywalker, Rey had a vision of Ochi of Bestoon, the Acolyte assassin who was hired by Emperor Palpatine to kill her parents and capture her.
Inquisitors
Also known as Jedi Hunters, the Inquisitorius was a group of Dark Side users founded by Palpatine following the end of the Clone Wars and the Galactic Empire's rise to power. The Inquisitors were trained by Darth Vader to locate and assassinate any Jedi who survived Order 66 along with other Force users who posed a danger to Palpatine like his former Sith apprentice Darth Maul. Inquisitors were also tasked with finding Force-sensitive individuals and killing those who refused to serve the Empire. In the animated TV series Star Wars Rebels, the Inquisitors were brought in to deal with the rebel cell on the planet Lothal.
Knights of Ren
The Knights of Ren were a group of masked Force-sensitive warriors who abided by the teachings of Ren, a Dark Side user who based his ideology on his red-bladed lightsaber and believed in "its nature, and nothing else." As such, the Knights of Ren didn't have a strict moral code and followed whatever path the Dark Side led them down in the wake of the Galactic Empire's defeat. Many years later, Ben Solo joined their ranks before becoming their leader, under the name Kylo Ren. The Knights of Ren were featured in the Star Wars Sequel Trilogy, where it was revealed they were really loyal to the resurrected Palpatine.
Nightbrothers
Before he became a Sith Lord, Darth Maul was born and raised among the Nightbrothers. A small group of Dathomirian Zabrak males who lived on the planet Dathomir, the Nightbrothers had a highly acrobatic fighting style and partly drew from the Dark Side of the Force. However, the more powerful ones could also undergo a dangerous ritual conducted by the Nightsisters, allowing them to better utilize the Dark Side. Maul and his sibling Savage Opress were further trained by Darth Sidious and Count Dooku's former apprentice Asajj Ventress, respectively, and teamed up in the later seasons of Star Wars: The Clone Wars.
Nightsisters
As deadly as the Nightbrothers were, it was the Nightsisters who reigned over Dathomir. Also known as the Witches of Dathomir, the Nightsisters drew their magic from the Dark Side and were able to tap into the dark energies surrounding their homeworld. However, their dealings with the Sith were treacherous at best, such as when Darth Sidious took away Maul -- the son of their leader, Mother Talzin -- in the years leading up to the Clone Wars, or when Dooku betrayed Asaaj, herself a Nightsister before becoming his apprentice. Ultimately, they two groups turned on each other, which led to the Nightsisters being destroyed.
Snoke & Kylo Ren
Ben Solo, the son of Han Solo and Leia Organa, was trained to become a Jedi by his uncle, Luke Skywalker, as part of a New Jedi Order. However, he was later corrupted by Supreme Leader Snoke and became Kylo Ren, a servant to the First Order. Interestingly, although Kylo Ren and Snoke resembled Sith Lords in the Star Wars Sequel Trilogy, they weren't technically part of the Sith and functioned as their own Dark Side faction. What's more, as Rise of Skywalker revealed, Snoke was a humanoid being created by a resurrected Palpatine to serve his purposes in the years after the Galactic Civil War.
Sorcerers of Tund & Yacombe
Little is known about the Sorcerers of Tund and Yacombe, other than they are both ancient groups who drew from the Dark Side of the Force. These factions are mentioned by the Jedi Master Sifo-Dyas in the 2019 audiobook Dooku: Lost Jedi, while he and Dooku are looking through the Bogon Collection -- a secret vault full of artifacts associated with the Dark Side -- in the archives of the Jedi Temple on Coruscant. It's possible these groups will be explored or touched upon in the upcoming TV series The Acolyte, which will examine the emerging threats from the Dark Side near the end of the High Republic era.