I am aware that Tenebrous has a short story centered around him, but for this poll, I am referring to full-length novels.

In that case, I’d have to go with Darth Cognus. I always fantasized how she managed to defeat Zannah and become the new Sith Master

Tenebrous, for those who don't know, was the master of Darth Plaugeis. He was very interesting as his version of the grand plan didn't really involve the force, but he wanted to use Bith science and Bith tech to make the Jedi obsolete so they would die out. He also created another secret apprentice named Darth Venamis, who would later be taken and killed by Plaugeis. Tenebrous would be a great choice for a book.

Tenebrous.

Cognus, mainly because it shows how the rule of 2 was not a linear path of apprentice offing master and adapting to defection scenarios. The split of the Banite Sith and the Millenial Sith would be an interesting area of exploration.

Darth tenbrus isn’t even in the book