The junior novelization of Star Wars: The Rise of Skywalker may have made an old Legends Sith Lord canon again. The Rise of Skywalker was essentially the Sith's last, desperate attempt to take control of the galaxy. Palpatine had survived his death in Return of the Jedi, his spirit retreating to a cloned body hidden away on the Sith redoubt of Exegol. This legendary world was steeped in the dark side, and had been a center of Sith power for untold millennia.
Lucasfilm used the opportunity to bring some ideas from the old Expanded Universe, now branded "Legends," back into canon. One of the statues showed the mask of Darth Nihilus, a Sith Lord from the Knights of the Old Republic games. According to the official Star Wars: The Rise of Skywalker Visual Dictionary, each legion of Sith Stormtroopers was named after an ancient Sith Lord, including the Revan Legion - confirming Darth Revan as canon as well. The film's novelization revealed even the medical equipment keeping Palpatine's cloned body alive was drawn from the EU: the crane facility was described as an Ommin harness, a clear reference to Tom Veitch's popular Tales of the Jedi: Knights of the Old Republic comics. It seems the trend continues in the junior novelization of Star Wars: The Rise of Skywalker, written by Michael Kogge.
Over on Reddit, user TheMastersSkywalker spotted a clear reference to another Sith Lord from the Expanded Universe: Naga Sadow, who played a prominent role in the Tales of the Jedi comics. In one early scene, Kylo Ren enters Exegol and passes the statues of the Sith Lords. Kogge describes it in detail:
Statues had been chiseled into the rock walls around him. He recognized some of them from his studies. Locphet. Mindran. Sissiri. Felkor. Sadow. All lords of the dark side. Their mouths did not move, but he heard their unintelligible whispers.
In Legends, Sadow originated from the Sith world of Ziost, and he became one of the most powerful Sith magicians of all time. He deliberately stoked war between the Sith Empire and the Republic as part of a power-grab, and came within a hair's breadth of conquering the entire galaxy, ultimately defeated by a rival Dark Lord of the Sith. Sadow placed himself in suspended animation, only to be awoken and subsequently slain by a would-be disciple. The junior novelization may not mention Naga Sadow's first name, but it's safe to assume it's referencing him. That said, readers should be wary of assuming every detail of Naga Sadow's life played out the same way. Certainly his spirit wound up on Exegol in the new canon.
The other Sith are unknown, new additions to Star Wars lore. It's interesting to note that Kylo Ren recognizes them, however, suggesting he has successfully acquired other Sith relics and possibly even Sith Holocrons. It will be interesting to see if any of these names turn up again, perhaps in the High Republic era soon to be explored in the books and comics.
Source: TheMastersSkywalker/Reddit