Palpatine's rise in the Star Wars universe is a meticulously planned bureaucratic coup d'état. Though the Galactic Senate scenes may not be as thrilling as space combat, watching Sheev Palpatine begrudgingly accept emergency powers while knowing it's exactly what plotted is a horrifying display of corruption. But Palpatine didn't come up with these techniques by himself, the real mastermind behind them is a little-known Sith Lord from before his time.
You've probably heard of Darth Plagues the Wise. Alluded to in Revenge of the Sith and expanded on in James Luceno's book, aptly titled Darth Plagueis. Plagueis was Palpatine's Sith master, who sought to find the secret to immortality but died at the hand of his apprentice. But what about Darth Plagueis's master, Darth Tenebrous?
Darth Tenebrous's first mention in the Darth Plagueis novel is cut short when he, like so many Sith Lords before him, is slain by his apprentice. While we didn't get to see much of him in action, his teachings through Plagueis show the true effect Tenebrous had on the future Emperor Palpatine.
Tenebrous was a Bith Sith, who had a very rationalist view of the Force. Like his apprentices after him, he used his powers as a tool to further his own agenda, but kept that agenda in line with the Sith Grand Plan of destroying the Jedi. He also had a public persona, Rugess Nome, and made a name for himself designing starships, all while keeping his identity secret.
This tool, manipulating those in power without letting slip his true intentions, was passed down to Darth Plagueis. He lived as Hego Damask of Damask Holdings and reaped the benefits of both Force powers and financial powers. In turn, the technique was passed on to minor noble Sheev Palpatine, who used his legacy of politics and became a Senator, while hiding his Sith background in plain sight.
Darth Tenebrous probably had a bigger influence on his Sith successors than they thought. In exclusive fiction published in Star Wars Insider #130, it was revealed that through his research of slow decaying midi-chlorians he called maxi-chlorians, he could transplant his consciousness into Plagueis. Leave it to an evil genius like Tenebrous to plan for his apprentice turning against him — this also feels like a possible explanation for the confusing reveal in Rise of Skywalker that killing your Sith master only causes them to take over your body.
However, Tenebrous and Plagueis both had the Force power of foresight, and in Plagueis' consciousness, Tenebrous foresaw Plagueis' death at the hands of Sidious. This devastated Tenebrous, who then tried to abandon the plan. However, his leaving Plagueis' consciousness robbed his apprentice of his future-seeing powers, all but guaranteeing his death at the hands of Palpatine.
So while Palpatine may not have met Darth Tenebrous, he owes him quite a bit. From the duplicitous way he enacted his master plan to his rebellion against his master, so much of his rise relies on this Sith Lord. It's almost a shame he couldn't get to see what evil his successor went on to achieve. Almost.