Star Wars has finally revealed what happened to a Sith cult active in the sequel era. Chuck Wendig's Aftermath trilogy was envisioned as an essential setup for the sequels, three novels set in the immediate aftermath of Return of the Jedi that revealed how the galaxy changed after the (apparent) death of Emperor Palpatine. The story culminated in the Battle of Jakku, the final defeat of the Emperor, and the end of the Galactic Civil War.
The books introduced a Sith cult known as the Acolytes of the Beyond. Formed by Palpatine loyalist Yupe Tashu, the members of the cult were not Force-sensitive themselves but they were obsessed with collecting Sith artifacts. They tended to wear ancient Sith masks, steeped in the dark side of the Force. Most readers assumed they were supposed to foreshadow Kylo Ren and his Knights of Ren. These theories are probably accurate, but Lucasfilm's plans weren't especially thorough, and the sequels pivoted in a different direction. The Acolytes of the Beyond became an irritating loose plot thread.
Adam Christopher's Shadow of the Sith finally explains what happened to the Acolytes of the Beyond. It seems that Luke Skywalker and his ally Lor San Tekka heard rumors they were amassing their own arsenal of Sith artifacts in an attempt to harness their dark power. Luke and Lor found the Acolytes to be a splintered, disorganized cabal, fascinated by the dark side but unable to truly wield its power. Luke shut the Acolytes of the Beyond down, taking their treasure trove of artifacts into his own keeping, and even redeemed one of their number.
Luke had underestimated the Acolytes of the Beyond, however, because one of their members was more fanatical than he had realized. In Star Wars, Kiza was a fanatical Sith acolyte and a leader among the Acolytes of the Beyond, one of the few members who potentially had a degree of Force sensitivity prior to donning an ancient mask that had once belonged to the Sith Viceroy Exim Panshard. The Sith Lord had survived death by transferring his essence into his mask, and when Kiza wore it she had fallen under his influence. Watching from afar, Palpatine saw Kiza and realized she could be a useful tool. As seen in Shadow of the Sith, he would ultimately use Kiza - and the specter of Exim Panshard - as a distraction. Luke had sensed a rising darkness in the galaxy, and he was even attempting to locate the Sith redoubt of Exegol. Palpatine arranged for Panshard and Kiza to cross Luke's path, and after their defeat, the Jedi Master wrongly believed the darkness had been vanquished.
This is the legacy of the Acolytes of the Beyond; there were Palpatine's pawns, used to distract Luke Skywalker. It will be interesting to see whether they do indeed have any direct tie to the sequel trilogy going forward. Perhaps Darth Vader's mask was among the artifacts Luke reclaimed from them, explaining how it ultimately fell into Ben Solo's hands. Still, for now, one Star Wars mystery seems to have been satisfactorily explained at last.
