Krayt or Sidious?
So let me detail the following passages for anybody who's having difficulties interpreting it. Tenebrous is talking about a calculation he came up with that showed the emergence of a "shadow" ruling over a group called the "One Sith." His plan is to get Plagueis to create Anakin, take control of Anakin as a baby and become the "shadow" tricking the Force in a sense. This ultimately fails as he sees ( or thinks he sees ) that the "shadow" in his calculation is Sidious. But was Sidious really the "shadow" of Tenebrous's calculation or was it Krayt? Both Krayt and Sidious have been described as shadows and both ultimately pursued goals of becoming the ultimate Force user of their era sharing power with no one ( Sidious sometimes contradicts this in various novels ). So was Tenebrous's calculation referencing Krayt or Sidious? Personally I'm unsure but given they were outright called the "One Sith" by Luceno I think his goal was to reference Krayt though leave the passage open to interpretation. If it was referencing Krayt from a lore perspective I'll definitely have to reevaluate my opinion of him as I have him around Vader level right now.
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While still merely an apprentice, his analysis had shown him the inevitable end of the Banite Sith and its preposterous Rule of Two. His calculations plainly indicated the coming of a shadow so vast it would darken the galaxy entirely- so vast it would mark the end of both Jedi and Sith as the universe had known them heretofore. The rise of the shadow would be the end of history itself.
Tenebrous had not the slightest doubt that the entire galaxy would measure time according to its arrival. Events would be marked by how far they had preceded the shadow, or by how long after it they followed.
"Though the exact nature of the great shadow remained occult, the remorseless logic of his extrapolation detailed the coming destruction of the Banite system, and the rise of what would become known as the "One Sith." One Sith! The conclusion was so obvious as to require no confirmation: one single Sith Lord would arise of such power that he'd have no need of any apprentice nor fear of the Jedi. He would take and hold the galaxy by his own hand alone. Without an apprentice- or a Jedi Order-to destroy him, the One Sith would rule forever!
A heady prospect, with only a single drawback:
Tenebrous was not to be that Sith Lord.
His own death was clearly foretold, entirely inevitable, and it would precede the rise of the shadow by decades. His fate was explicit in the numbers, and numbers do not lie. However-as Tenebrous came eventually to realize over his many years of research, contemplation and calculation-it might be possible for the numbers in question to be, well, deceived....
The key, he'd discovered, lay in an obscure legend obliquely referenced in the Journal of the Whills, about a hero fairly typical in most cultures-the sort of promised future savior who appears in the foundational myths of nearly every developed society. What distinguished this particular savior from his run-of-the-mill equivalents was that he, according to four of eleven possible translations, was to be "born of pure Force." After three standard years devoted specifically to exploring all possible permutations of the interpretation, Tenebrous determined that such a birth was indeed possible, at least metaphorically- "born of pure Force" could be read as indicating the creation of a living being through direct manipulation of midi-chlorian processes in an already living being.
And further, as Tenebrous discovered with rising excitement, such a being s Force potential might be limited not by its creator's own midi-chlorian count, but instead only by its creator's level of discipline and attention to detail. Indeed, his calculations indicated a range potentially far beyond his own. With proper execution, the "savior" might have a midi-chlorian count as high as fifteen thousand!
Perhaps even more.
It might be possible to create a being with the greatest Force potential ever recorded!
And-by the application of his own suitably subtle variation of the ancient Sith brute-force essence transfer-Tenebrous could ensure that his own consciousness would be present at the creation of this being, this savior, this Chosen One. And, at the moment of creation-long before the Chosen One could hope to resist- Tenebrous would seize it. Would become it.
With this single stroke, decades after his body's death, he would become the most powerful Force-user in the history of the galaxy." - The Tenebrous Way.
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"Now Tenebrous touched upon his apprentice's powers of foresight, which were also vastly more developed than Tenebrous had believed. For a moment. Tenebrous found his perception cast far forward in time-to Plagueis' own death at the hands of his apprentice, who was himself visible only as a smear of darkness....
A shadow!
For an instant, Tenebrous felt the death anguish of Plagueis.... and felt the searing agony Plagueis felt.... at his failure to have ever created the Force-user Tenebrous was to become! He would allow his own apprentice to kill him too soon...." - The Tenebrous Way.