If "Sion" refers to a Sith Lord from Legends, may connect to the Old Republic era.

Source: https://screenrant.com/ahsoka-eye-of-sion-star-wars-legends/

Outside of this, Vader wasn't seen as a powerful, deadly Sith Lord.

Source: https://www.cbr.com/ahsoka-darth-vader-real-version/

Part of the fan perception of the character shifted in recent years, however, when a certain theory circulated online stating that Jar Jar Binks was in fact a secret Sith Lord.

Source: https://www.cbr.com/jar-jar-binks-actor-ahmed-best-mandalorian-season-3/

Star Wars: Episode I - The Phantom Menace, Jedi Master Qui-Gon Jinn was murdered by Darth Maul, a new Sith Lord who himself was defeated by Obi-Wan Kenobi.

Source: https://screenrant.com/star-wars-george-lucas-sith-rule-of-two-misunderstood/

The story of the Sith Lord who learned to manipulate Midi-chlorians to create life, and the Apprentice who killed him in his sleep, they speculate, had to be more than it seemed.

Source: https://screenrant.com/star-wars-anakin-father-emperor/

The tale of Exar Kun, a Jedi turned Sith Lord, adds a layer of irony to Niman's legacy.

Source: https://gamerant.com/star-wars-lightsaber-form-niman-explained/

The term was used by the ancient Sith, and preserved by Darth Bane, a Sith Lord who survived the Jedi-Sith War.

Source: https://gamerant.com/star-wars-why-do-sith-name-themselves-darth/

With the demise of his apprentice, Darth Maul, Palpatine had Dooku prove himself as a Sith Lord by striking down his old friend, Jedi Master Yaddle.

Source: https://screenrant.com/star-wars-before-phantom-menace-timeline/