Short answers, pulled from the story.
Star Wars: Tales of the Jedi is a series of comic books published by Dark Horse Comics between 1993 and 1998. The series covers two ancient conflicts in the Star Wars universe: the Great Sith War, set around 3,996 years before the Battle of Yavin, and the Great Hyperspace War, which took place a thousand years earlier. It was the earliest chronological Star Wars story until the Dawn of the Jedi series was published.
The series was written primarily by Tom Veitch and Kevin J. Anderson. Veitch wrote the earlier arcs including Knights of the Old Republic and The Freedon Nadd Uprising, while Anderson wrote or co-wrote Dark Lords of the Sith, The Sith War, The Golden Age of the Sith, The Fall of the Sith Empire, and Redemption.
The central characters include Ulic Qel-Droma, a Jedi who falls to the dark side and becomes apprentice to Exar Kun, the Dark Lord of the Sith. Nomi Sunrider is a key Jedi hero who ultimately severs Ulic's connection to the Force. The ancient Sith Lord Naga Sadow and his rival Ludo Kressh drive the Great Hyperspace War storyline.
The Tales of the Jedi comics are no longer part of the official Star Wars canon. However, some elements from the series have been brought into canon: Exar Kun appeared in promotional materials for the 2018 film Solo: A Star Wars Story, and the planet Ossus, introduced in The Freedon Nadd Uprising, was referenced in the 2022 canon novel Shadow of the Sith and depicted in The Book of Boba Fett.
The series contains six story arcs plus one sequel arc. The main arcs are Knights of the Old Republic (1993-94), The Freedon Nadd Uprising (1994), Dark Lords of the Sith (1994-95), The Sith War (1995-96), The Golden Age of the Sith (1996-97), and The Fall of the Sith Empire (1997). Redemption (1998) is a five-part sequel set ten years after the Sith War.
Exar Kun turned to the dark side after leaving his master Vodo-Siosk Baas and seeking forbidden knowledge. The spirit of Freedon Nadd tricked him into accepting the dark side on Korriban, and on Yavin IV Nadd gave Kun an ultimatum to embrace the dark side or die. The ghost of ancient Sith Lord Marka Ragnos later declared Kun the Dark Lord of the Sith when Kun and Ulic Qel-Droma clashed on Cinnagar.