When Obi-wan goes to save Leia from the Third Sister, he discovers the Crypt - full of bodies encased in amber (or some such)
Who are these people?
The Wikipedia page reads:
Having escaped Vader on Mapuzo, Kenobi and Tala arrive at a Path facility on Jabiim. Meanwhile, Leia, being held in Fortress Inquisitorius, the stronghold headquarters of the Inquisitors located on Nur, is now being interrogated for details on the Path by Reva. Kenobi and Tala plan to infiltrate the fortress to rescue Leia. Once inside, Kenobi discovers a trophy vault filled with the preserved corpses of Jedi who had been captured and killed, including a Youngling.
Further proving the fact is the presence of Tera Sinube, who is elderly jedi who most prominently appeared in the clone wars
So though certainly not sure, but we can say that it was either a tomb of the jedi killed in order 66 by the inquisitors and the inquisitors kept their bodies as trophies or the tomb of the enemies of the empire overall and they were just there as trophies for the inquisitor's.
Another theory quoted by Screenrant -
The purpose of the Inquisitors preserving Jedi corpses is unclear in the episode, but other Star Wars properties imply their potential uses. In Star Wars Rebels season 1, the Grand Inquisitor uses the corpse of Luminara Unduli to lure Jedi (or potential Jedi) into a deadly trap. The Mandalorian strongly suggests that the blood of powerful Force-sensitives is being harvested for experiments, potentially the creation of Force-sensitive strand-casts (such as Snoke) or the eventual resurrection of Emperor Palpatine.