Kreia confirms that Darth Nihilus is the most powerful Sith to ever exist
The revelation from Kreia:
"He [Darth Nihilus] cares nothing for the Sith or its teachings… or the Jedi. And when the Jedi are dead, he will feed on the galaxy, the Republic, and eventually, consume the Sith as well. "
- Kreia
Kreia was the Dark Lord Traya, who actually knew many things about the true Sith Empire and knew just how powerful they were:
"If you were to face an ancient Sith Lord in combat, you would learn that we are as children playing with toys compared to the prowess of the old Masters."
- Kreia
Traya sees the True Sith Empire beyond the Outer Rim in the Unknown Regions, and foresees their recapturing of Korriban three centuries later. Kreia also sees Revan's choice to travel into the Unknown Regions, and his decision to leave behind Bastila along with the rest of his party and the Ebon Hawk. Finally, she foresees Surik's decision to follow him without any of her loved ones:
Kreia learned of Nihilus's nature and the truth of his powers through the Sith Hermetica she read on Malachor V.
She knew of the true Sith Empire when almost no one else did, except for her student Revan.
Kreia learned from the Sith Hermetica that Nihilus's drain had no defense to it and that he could kill an entire planet in seconds.
Kreia knew of all the times the Republic would fall:
"The Republic will fall, as it always has. A fall that will take millennia. Under the care of the herds of Ithor, the surface of Telos will bloom again, and its golden fields shall again harbor scientists and thinkers. And complacent and peaceful, it will forget the time that Saul Karath orbited it and brought fire to its skies. But it shall be a homeworld again to others, who will stretch out across the galaxy and bring life." "
Kreia also knew about Darth Sidious:
"And others have mastered the more subtle work of politics, persuasion. Do not doubt that a galaxy may be conquered with words, a Republic overthrown, and an empire made."
Kreia knew of the true Sith - she knew their power.
Through the Sith Hermetica, she knew of Nihilus's power - and she knew he would kill them all.
"He [Darth Nihilus] cares nothing for the Sith or its teachings… or the Jedi. And when the Jedi are dead, he will feed on the galaxy, the Republic, and eventually, consume the Sith as well."