Krayt's rise to power
Krayt's rise to power explained here:
From Star Wars: The Essential Guide to WarfareDarth Bane's Rule of Two decreed that there would only be two true Sith at a time—a master and an apprentice. This practice allowed the Sith Order to survive during the Republic's final millennium, with Darth Sidious eventually engineering the downfall of the Jedi Order and reestablishing Sith rule of the galaxy.
Sidious trained and created a number of Dark Side Adepts, but did not overturn the Rule of Two—his dark side hopefuls were denied the status of Sith or access to the Order's secrets, leaving Sidious and Darth Vader as the galaxy's only Sith Lords. Lumiya, who claimed the Sith mantle after Sidious and Vader died at Endor, also obeyed the Rule of Two, as did Jacen Solo when he became Darth Caedus. When Caedus died and his apprentice Tahiri Veila turned back to the light, the succession of Sith was believed broken, and the Order thought finally extinct.
But the Sith would be reconstituted from a surprising source. In 5000 BBY, a Sith starship in service to Naga Sadow crashed on the hidden world of Kesh. The survivors became the Lost Tribe of the Sith—an Order that grew and flourished on Kesh even as the Sith were forced into hiding in the larger galaxy. In 41 ABY, a Sith meditation sphere ended the Lost Tribe's millennia of isolation, informing them of the ancient Sith Empire's destruction at the hands of the Jedi, and preparing them to take their revenge.
The Lost Tribe ultimately failed to overthrow the Jedi, but the Sith survived their downfall, too, and their glory would be rekindled by one who had watched their triumph and ruin. Darth Krayt had begun his life as A'sharad Hett, a human from Tatooine raised as a Tusken Raider and trained as a Jedi. Hett survived Order 66 and abandoned the Jedi, becoming a bounty hunter and wanderer in a war-torn galaxy. But though he thought himself done with the Force, it wasn't done with him. In the Unknown Regions, he was captured by the Yuuzhan Vong and tutored by the mysterious Fosh named Vergere, who remade his body through Vong experiments and taught him to open himself to the dark side. Taking the name Darth Krayt, he hid away on the Sith tombworld of Korriban. There he would draw lessons from the extinction of the old Sith Order under Darth Caedus and the Lost Tribe’s defeat.
Rather than the Rule of Two, Krayt decreed that the Sith would obey the Rule of One, following the dictates of the Dark Lord who stood atop the Sith hierarchy. No longer would the Sith seek power as its own reward; now they would work to use that power toward a greater goal. His life span extended by the Vong experiments, Krayt secretly rebuilt the Order's numbers, and waited for a chance to cast down the Jedi and reclaim power. He found it, ironically, in a Jedi attempt to heal the wounds of the Yuuzhan Vong War.
After invading the galaxy, most of the Yuuzhan Vong settled on the living world of Zonama Sekot. But a few chose to dwell elsewhere. Decades later, the descendants of these Vong often found themselves mistreated and reviled—vengeful Bothans hunted them, while other species refused to deal with them. This troubled Kol Skywalker, leader of the Jedi Council—it was the Jedi’s responsibility to seek justice for all members of galactic society, even the very least. Particularly the very least, Skywalker thought. After meditating on the problem for many months, Skywalker had an idea.
In 122 ABY Skywalker and the Vong master shaper Nei Rin tested using Vongforming techniques to terraform a desolate section of the planet Ossus. When Ossus became a lush green world once again, Skywalker and Rin persuaded the Galactic Alliance to let Vong shapers heal a hundred of the worlds devastated by the Vong invasion. This promising undertaking was called the Ossus Project.
But something disastrous happened—at first the worlds healed, but then plants and animals began sprouting hideous bony spurs akin to those that erupted from Vong captives in the original invasion.
The Jedi defended the Vong, swearing that the project had been sabotaged, and convinced the Galactic Alliance to protect them. That led to a succession movement—and hard-line Moffs in the Empire (now led by Roan Fel) saw a chance for the Empire to reclaim its supremacy. Over Fel's objections, in 127 ABY they left the Alliance, recruited disaffected worlds to their cause, and declared war.
A year into the war, the Sith—the Ossus Project's saboteurs—approached the Moffs and offered an alliance, which was accepted. Over the next two years, the Sith and Fel's Empire won victory after victory. At the Battle of Caamas, Grand Admiral Morlish Veed and his Imperial fleet crushed the Alliance's forces. They surrendered, with the exception of the Duros admiral Gar Stazi, who mustered what ships he could and fled. The Alliance surrendered days later, as did some members of the Jedi Order, who joined Fel's Imperial Knights. The other Jedi withdrew to Ossus, where they were massacred by the Sith and Imperial forces. Soon after that, the Sith led a coup against Emperor Fel. The Sith Lord Darth Krayt took the throne on Coruscant as head of a new Sith Empire that once more ruled the galaxy.
Fel and his Imperial Knights retook Bastion, and defended a portion of the New Territories against the ruthless Sith Empire, beginning a new galactic civil war. Admiral Stazi's forces, meanwhile, began a guerrilla campaign against the Sith. And the scattered Jedi Knights sought to rebuild their Order in secret.
The Jedi defended the Vong, swearing that the project had been sabotaged, and convinced the Galactic Alliance to protect them. That led to a succession movement—and hard-line Moffs in the Empire (now led by Roan Fel) saw a chance for the Empire to reclaim its supremacy. Over Fel's objections, in 127 ABY they left the Alliance, recruited disaffected worlds to their cause, and declared war.
A year into the war, the Sith—the Ossus Project's saboteurs—approached the Moffs and offered an alliance, which was accepted. Over the next two years, the Sith and Fel's Empire won victory after victory. At the Battle of Caamas, Grand Admiral Morlish Veed and his Imperial fleet crushed the Alliance's forces. They surrendered, with the exception of the Duros admiral Gar Stazi, who mustered what ships he could and fled. The Alliance surrendered days later, as did some members of the Jedi Order, who joined Fel's Imperial Knights. The other Jedi withdrew to Ossus, where they were massacred by the Sith and Imperial forces. Soon after that, the Sith led a coup against Emperor Fel. The Sith Lord Darth Krayt took the throne on Coruscant as head of a new Sith Empire that once more ruled the galaxy.
Fel and his Imperial Knights retook Bastion, and defended a portion of the New Territories against the ruthless Sith Empire, beginning a new galactic civil war. Admiral Stazi’s forces, meanwhile, began a guerrilla campaign against the Sith. And the scattered Jedi Knights sought to rebuild their Order in secret.