Light Sith or light-sided Sith were adherents of the Sith that rejected the dark side of the Force in favour of the light.
During the Cold War a group of light-leaning Sith formed within the ranks of the Sith Empire. The Sith Lord Cendence was hand-chosen by the Dark Council to hunt down and destroy any Sith not deemed pure.
During this time, Sith apprentice Jaesa Willsaam was ordered by her Master to track down the renegade Sith. Jaesa succeeded in her mission and slaughtered them all. A while later, she found a gravely injured Light Sith. Before he died, he told about Cendence. Jaesa found Cendence, but after realizing that he killed any Sith he pleased, she killed him. Caliqu, another Sith Lord, then gave Jaesa his position.
Centuries later a Sith Lord named Darth Gravid also came to the conclusion that pure devotion to the dark side would curse the Sith of his era to the same fate as their predecessors, and sought a new philosophy in which the Sith would follow the light side of the Force, strengthening themselves with compassion and love, as their Jedi counterparts did. Sealed away in his Jaguada bastion Gravid set about amassing centuries worth of Sith lore that had been passed down from Master to apprentice since the time of Darth Bane, including: holocrons, crystals, scrolls, tomes, amulets and all other manner of artifacts, and proceeded to burn them, seeking to ensure that future generations of aspiring Sith remained committed to the path he had laid before them.
Unfortunately, Gravid's apprentice Darth Gean did not share her Master's vision. Horrified by the destruction of the Sith's treasures she threw herself at her Master, attacking Gravid with her bare hands, and after a violent struggle, succeeded in killing him, bringing an abrupt end to his plans. Although many valuable artifacts and teachings were consigned to the flames, including the coveted technique of essence transfer, Gean did manage to preserve enough material with which to instruct her own apprentice, ensuring the continuation of Darth Bane's original intentions: Unfaltering allegiance to the dark side.
Darth Plagueis later recounted the events of Darth Gravid's demise in regards to seeking out the lost method of immortality. Likewise, Plagueis would do something similar in spirit to Gravid—he attempted to end the Rule of Two, only to end up being murdered by his apprentice, Darth Sidious as a consequence, who felt that ending it was against the nature of the dark side.
Should Jaesa Willsaam remain on the light side, she instead seeks out the Light Sith in hopes of alliance. Her first attempts are unsuccessful as they are suspicious of her and attack, forcing Willsaam to kill in self defense. Surviving as a Light Sith requires a healthy level of suspicion, as her master is well aware, and her approach was too naive to be believable. Future attempts work much better.
It is possible for a player Sith Warrior or Sith Inquisitor to be a Light Sith. In the Inquisitor's case, their approach leads them to earn the title of Darth Imperius, due to their light-sided actions being viewed as pragmatic acts that serve the Empire in the long run. Notably, if the player creates a level 70 Inquisitor in order to start the game at the Knights of the Fallen Empire expansion, their Sith title will default to Imperius even though the game otherwise assumes they made all dark-side choices.
Originally, in Star Wars: The Old Republic, all light side Sith the player encounters throughout the game wielded purple-bladed lightsabers. This was due to a gameplay mechanic which restricted the use of red-colored crystals to darksiders only. This limitation has since been removed, however.