Ku'ar Danar was part of a Jedi exploration team which discovered Korriban, the homeworld of the Sith species. But contrary to his comrades, Danar stayed longer on the planet to study the Sith use of the Force. Danar then succumbed to the dark side but didn't take an active part in the conflict which occurred sometime later. In fact, the fallen Jedi preferred conducting an experiment in the Nilrebmah system. After subjugating the civilization on Nilrebmah II, he obliged the population to build a gigantic monolith on Nilrebmah XIII using living, Force-sensitive stones from Nilrebmah X.
His experiment succeeded, as he managed to move an entire planet from his orbit with only the Force, but all life on it was annihilated as well as his own corpse. However, his spirit stayed prisoner in the monolith, waiting for his saviors.
After the Hundred-Year Darkness, Danar disappearance was noticed and many Jedi suspected him to have pulled the strings of the war, especially since they found a manuscript in the archives of the Exiles which partially described the experience of Danar and his "success".
One year before the Conclave on Deneba, the monolith was discovered by Sli'Lon Tahar, a Jedi master searching for proof of the Sith survival after the Great Hyperspace War. Tahar didn't have the time to reveal its position as the Krath attacked the conclave. Two years after his discovery, Tahar was killed by the Sith Lord Ulic Qel-Droma in the Battle of Foerost. Before dying, he had the time to reveal the existence of the monolith to adventurers came from the future, but also unintentionally to Qel-Droma himself.
Later, the mysterious adventurers found on Ossus erroneous information on the presumed Sith Lord, seconded by a Jedi archivist and his apprentice, Dace Vinagar. The Order also mistakenly thought that he had formed Naga Sadow to the manipulation of the Force before he became Dark Lord of the Sith, and that he instructed some of the most feared geneticists from the time. They also read fragments of the manuscript discovered after the Second Great Schism, and determined the localization of the monolith in the Nilrebmah system. Fifteen days later, they explored the system with the assistance of Vinagar, only to be attacked on Nilrebmah XIII by dark side creatures. Ku'ar Danar, sensing the presence of Vinagar, lured him in the depth of the planet. The Jedi apprentice finally betrayed his comrades and tried to kill them, then followed the call of who he thought being a "Sith Lord".
Ku'ar Danar's monolith on Nilrebmah XIII.
Danar slowed the adventurers by reanimating the skeletons of the civilization he destroyed, gaining time for Dace Vinagar to come inside the monolith. He then empowered the apprentice with his own Force abilities, allowing him to unleash dark side beasts on his pursuers. Those beasts gave Vinagar just enough time to finish an old ritual and activate the monolith, liberating the spirit of Ku'at Danar. Cursing the Force spirit of the Fallen Jedi when his body started to consume itself, Vinagar let his rage amplify the power of the ritual and was finally changed in a giant Hydra moved by pure energy.
The entire planet started to collapse, as it moved toward Nilrebmah XII, and the hydra was shot down by a Krath starship commanded by Ulic Qel-Droma who searched the tomb of Ku'ar Danar. At the moment where the two planets collided, the adventurers entered hyperspace, only to find themselves in capacity to return in their era. What they didn't know is that the spirit of Danar followed them and gave them this opportunity himself.
Danar was a solitary and discreet man, staying on Korriban when his companions left the planet and seemingly staying out of the conflict which divided the Jedi Order. But many of the Jedi of the era started to think that he was at least partially responsible of the situation, thus knowing him as a manipulative individual. This trait can be perceived in the use and the destruction of an entire civilization for a personal, useless experimentation. This total disinterest for power frightened the Jedi, who seen him as a fundamentalist theorist with no thinking of the consequences of his action.
Ku'ar Danar was a very powerful Force-sensitive, and was able to manipulate the Force and the dark side to a high level of competence. Even as a Force spirit, he managed to unleash beasts on the intruders and reanimated millennia old corpses.
He was also able to furnish to the adventurers from the future the surplus of energy necessary to the time travel.
When he was alive, he successfully dominated an entire civilization, forcing them to erect a gigantic monolith, and was able to use the structure as an amplifier for his Force experimentations.
Ku'ar Danar was created by Philippe Rat and Mehdi Sahmi for the ambiguously canon roleplaying scenario "Le Facteur X" which appeared in the French magazine Casus Belli 115.
The article preceding the scenario is inconsistent with the actual EU, but the scenario himself, even based on now erroneous data, presents few if any problems. Jedi discovering Korriban and the Sith before the Second Great Schism can be considered as an error, but with the possible manipulation of the Dark Jedi by Ku'ar Danar, it is possible for him knowing about the planet and the species without telling them the truth. From this perspective, the arrival of the Exiles on Korriban can perhaps be viewed as non-accidental, even if the Exiles didn't know it.
However, the fragments of the manuscript in the scenario specifically mentioned the "Sith". In the context of writing of the article and scenario, the Sith are explicitly the Dark Jedi who later returned on Korriban. In the context of the actual EU, it can be speculated that at least one Dark Jedi knew about the studies of Ku'ar Danar on Korriban and knew the word "Sith" before the Exiles arrived on Korriban. The more recent introduction of the Exile Sorzus Syn and her partial knowledge of the Sith and of the location of Korriban could help explaining those discrepancies.
Ku'ar Danar is considered as an ancient Sith Lord in the story, but can't be one, since the Exiles were the first "non-Sith" to claim the title. But it can be noted that in the entire scenario, only Jedi and Fallen Jedi call him a "Sith". Thus, the title can be explained as late postulate made by the Jedi on the assumption that he was partly responsible of the Hundred-Year Darkness. To support this, it can also be noted that the archivists and librarians on Ossus gave to the adventurers information which was erroneous, even in the context of the scenario. For example, Ku'ar Danar can't have formed Naga Sadow even as a Force spirit since he is trapped on Nilrebmah XIII. However, this false connection implies that the Jedi knew Danar as a powerful sorcerer, as was Sadow.