A dark side of the Force, Lucasfilm has published a book called Secrets of the Sith that purports to be a collection of notes from Emperor Palpatine himself.
Naturally, Secrets of the Sith also reveals a great deal about Palpatine's own master, Darth Plagueis. Plagueis was previously mentioned in lying to Anakin about Darth Plagueis; he really had learned how to prevent another person dying, and years later the Emperor would expand upon this technique to master the art of Essence Transfer, allowing Palpatine's spirit to endure after his own body had been destroyed.
It seems Darth Plagueis was particularly interested in the legend of the Force Dyad, which he believed to be a source of power greater than life itself. "My master, Darth Plagueis, attempted to forge such a bond with me," Palpatine records. "Though he was wise in the ways of the Force, he proved unworthy of the task. I, too, attempted to facilitate such a connection with my apprentice Anakin Skywalker. Yet even with the so-called Chosen One at my side, the balance we shared pulled against the perfection of the dyad." It is ironic that, toward the end of his life, Emperor Palpatine would finally encounter the phenomenon the Sith had coveted so.
The Sith Rule of Two is related to the Dyad, and it is likely every Sith Lord attempted to create such a bond with their apprentice. They failed, likely because the novelization of create a Force Dyad, however much they coveted it.
Darth Plagueis may well have been particularly interested in the Force Dyad. Given the techniques he mastered, he seems to have been obsessed with the idea of immortality; and he would have been aware of a Force Dyad were possessed of a distinctive Force Heal power, the one used by Rey and Kylo Ren during Star Wars: The Rise of Skywalker. Truly the Dyad was indeed a power greater than life itself - even if it was inaccessible to Star Wars' Sith.
