Hundred-Year Darkness
The Hundred-Year Darkness, which is also known as the Hundred-Year War, was a prolonged period of conflict. It commenced around 7000 BBY and continued until approximately 6900 BBY. This era was marked by the emergence of monstrous armies created by a group of Dark Jedi to wage war against both the Jedi Order and the Galactic Republic. This period of darkness began three years following the Dark Jedi's departure from the Order during the Second Great Schism. After a century of battles, the Dark Jedi were defeated at the Battle of Corbos, leading them to conquer Korriban and establish the Sith.
Origins
The seeds of the Second Great Schism were sown when a faction of Jedi began to explore and utilize advanced applications of the Dark side of the Force. Through their increasing reliance on the dark side, these Jedi discovered that the Force, when wielded with sufficient intensity, could manipulate life itself. Harnessing their newfound abilities, these Knights conducted experiments on living organisms, giving rise to monstrous beings that served as extensions of their will. These powers were met with horror by other Jedi. Although the initial rebels, known as the Legions of Lettow, had suffered defeat in the First Great Schism, their legacy endured, paving the way for a second group of dissidents to emerge, spearheaded by Ajunta Pall.
All major conflicts are fueled by passion, and this one was no exception. Determined to safeguard their newly acquired abilities, this group of Jedi endeavored to persuade their counterparts to embrace the Dark Side rather than fear it. However, their efforts proved unsuccessful. As each side remained steadfast in their convictions, disagreements escalated into open conflict. Consequently, the rogue Jedi were exiled along with their teachings, marking the commencement of a century-long war.
Warfare
During the course of the war, the Jedi constructed the Prism, a clandestine prison facility known only to members of the Jedi High Council.
As the war progressed, Sorzus Syn engineered a variety of monstrous creatures, including Shamblers, Howlers, Pit Horrors, and ultimately, Leviathans, beings capable of draining life essences into blister-traps located on their backs.
The final confrontation involving the Dark Jedi took place on the planet Corbos. Due to internal strife and infighting, the Dark Jedi were ultimately defeated.
Banishment
Following their defeat on Corbos, the surviving Dark Jedi were apprehended by the Jedi. These Exiles, as they became known among the Jedi, were disarmed, stripped of their navigational tools, and forced onto unarmed galleons, before being banished beyond the boundaries of known space.
With the defeat of the Dark Jedi, the Hundred-Year Darkness came to an end, and the Jedi had successfully navigated the Second Great Schism.
A New Home
Driven by tales of the Kingdom of Sith, Sorzus Syn led the Dark Jedi to Korriban. Upon arriving on the planet, they discovered that the rumors were true: it was inhabited by a humanoid species known as the Sith. Hakagram Graush, the King of the Sith, was initially taken aback by the appearance of the outsiders. However, after witnessing their powers and advanced technology, the king welcomed the exiled Jedi to his world. This decision, however, would lead to Graush's undoing. Graush's Shadow Hand conspired with the newcomers, betraying his sovereign to the Dark Jedi. Utilizing his mastery of the Force, Ajunta Pall decapitated Graush, convincing the Sith that the Exiles were gods. The Exiles then ascended to become the rulers of the Sith, adopting the title of Lords of the Sith. Ajunta Pall thus became the first Dark Lord of the Sith.
From the Sith, Sorzus Syn learned of the prophecy of the Sith'ari. The Sith'ari was prophesized to be a perfect being destined to lead the Sith. According to the prophecy, this individual would rise to power, ultimately destroying the Sith, but in doing so, would make them stronger than ever before. Syn recognized a striking parallel between this prophecy and the Jedi legend of the Chosen One, leading her to speculate that she herself was the Sith'ari.
The Return
After becoming Sith Lords, against the orders of Ajunta Pall, some of the Exiles took their arrival ship, new weapons, Massassi warriors, and their newfound secrets of Sith alchemy in an attempt to destroy the Jedi, only to be destroyed themselves. This gave the Jedi the knowledge that Sith Space existed, and that the Exiles had survived.
Aftermath
Over time, as interbreeding between the Exiles and the Sith occurred, the term "Sith" evolved to encompass not only the species but also their fallen Jedi masters. Initially, the Sith wore simple attire, but with the influence of the Exiles, more elaborate clothing became commonplace.
Behind the Curtains
The initial published reference to fallen Jedi becoming the rulers of the "Sith civilization" appeared in Tales of the Jedi – The Freedon Nadd Uprising 1. This information was reiterated in Tales of the Jedi – Dark Lords of the Sith 1. The first use of the name "the Hundred-Year Darkness" was in Tales of the Jedi – Dark Lords of the Sith 3, within a list of events associated with the Dark Side, though without further elaboration.
Tales of the Jedi – The Golden Age of the Sith 0 mentions that the "First Great Schism" was followed by "a century of bloodshed" that ended with the Dark Jedi faction being exiled and becoming the first Sith Lords. In Tales of the Jedi – The Golden Age of the Sith 2, the Dark Lord of the Sith Marka Ragnos states that the ancestors of the Sith Lords were exiled as a result of the "Great Schism," implying that because his ancestors were exiled at the end of the "First Great Schism", he's unaware that there was a "Second Great Schism."
Arden Lyn's biography in "The Emperor's Pawns", a collection of biographies published in Star Wars Gamer #5 (2001), indicates that the "First Great Schism" occurred around 25,000 BBY. The timeline at the beginning of The New Essential Guide to Characters (2002) notes that "circa 7,000 BBY", "The Jedi Knights lead the Republic through the Hundred-Year Darkness", but provides no additional details. The New Essential Guide to Characters also repeats the information about the "First Great Schism" from Tales of the Jedi: Golden Age of the Sith and the year from "The Emperor's Pawns" in the timeline at the beginning of the book and in the character entry for Naga Sadow.
The New Essential Chronology (2005) reaffirms that the Hundred-Year Darkness occurred around 7000 BBY. It also reveals that this event is the same as the "hundred years of bloodshed" mentioned in Tales of the Jedi: Golden Age of the Sith, and that it followed the "Second Great Schism," while the "First Great Schism" was retconned to be a separate event. When asked about this retcon, Daniel Wallace, the author of The New Essential Chronology, explained that it was based on "LFL internal documents" that dated Dark Jedi contact with the Sith species to around 7000 BBY. Leland Chee stated that this retcon was to "correct the the (sic) original discrepency of the timing of the Great Schism introduced by The New Essential Guide to Characters", but didn't say what source The New Essential Guide to Characters created a discrepency with.