Darth Rook

"Who is Darth Rook?"

"He's known as The Loyalty and also quite powerful to the Sith linage.""- Sidious explain to Vader about him."

Darth Rook was an unknown Force-Sensitive humanoid https://starwars.wikia.com/wiki/Dark_Lord_of_the_Sith who serve under the linage of the Rule of Two after the Jedi won their victor against the Sith in 1032 BBY before Darth Bane, the last living Sith who create the Rule of Two. He was known as Darth Rook the Loyalty.

Biography:
Darth Rook was known as The Loyalty who is very loyal throughout in the Sith history. Rooks was so powerful to use his manipulation to trick any species from his loyal word to outrank them from those who truly believe his rightful word to convince them from their needed. He was powerful to compare his Master who taught him about the Force and understand about power from his wild dream, but he is smart and clever to outranking his Master by using his loyalty manipulation to make his a puppet to get closer and kill him from his unexpected attack that Rook was truly loyalty to the Sith.

He build his Holocron to all future Sith Generation before his time come to meet his fate by his own apprentice and his apprentice was also taught his own student as well. So either one of them will find his Holocron to understand the power of Loyalty, so that loyalty is the key to find their own Sith Master and outrank any highest ranking from their greed ambition.