Poll
57%
43%
"For a millennium, the Sith maintained the order in secrecy, passing down their evil heritage. As they gained knowledge of the dark side of the Force, their powers increased with each generation."
Source: Episode 1: The Phantom Menace Scrapbook
Ultimately, Bane's plan produced more powerful Sith Lords with every generation.
Source: Force & Destiny
Okay...Maul is a 'Sith'.
Anyway, Tenebrous had access to all of the Sith holocrons Darth Bane had at his disposal in addition to the others, and new Sith discoveries, contrived in-between those 28 generations of Sith. Additionally, Bane's powers are passed down with each Sith, so we can assume that Tenebrous is at the very least just as powerful as Darth Bane:
- Book of the Sith
"Darth Bane's Rule of Two was a keystone of the Sith Order for centuries. The Battle of Ruusan, nearly a millenium ago, would have ended the Sith Order had Darth Bane not reconstituted it as a diarchy operating from the shadows. His writings became a part of the Sith Archives passed down from master to apprentice for generations."
[...]
"Under the tutelage of Darth Plagueis I inherited the Sith Archives - more than a thousand years' worth of teachings passed in secret from master to apprentice."
-- Darth Sidious
- Book of the Sith
And of course Tenebrous had access to all of this and presumably more:
More than a century before, when Tenebrous had been but a Sith apprentice himself, the magnificent computational power of his Bith brain had led him far beyond the simplistic Force studies imposed on him by his Master.
- The Tenebrous Way