JJ Abrams' penchant for so called 'fanservice' is the best thing about this movie in the sense that it tries to 'get' the characters right largely by way of blatantly disregarding and negating what the previous episode did. It genuinely feels like he's been listening to objections fans had, although when one looks back at the previous episode, can easily see how the characters and story reached a stage it was quite difficult to build up from, and that undid a lot of what JJ himself did in episode VII.
It happens also to be one of the worst parts of the movie, though. Sadly, the need to rebuild from the hiccup episode VIII turned out to be made this movie need to be at least a couple of movies, rushing character arcs with quite a bit of a rollercoaster. Logic problems aside that I am sure will be somewhat and somehow explained in non-movie material (the Empire in any incarnation always seemed to have in books and comics absurdly powerful trump cards that happened to just sit there), the movie itself is one MacGuffin after another, with some visually stunning scenes but a rather disjointed editing. The effort to give some depth to the trilogy-specific characters making up for lost time is genuine, with mixed results: Finn gets heroic moments and his force sensitivity is made manifest - both traits he were robbed of in VIII -, Poe completes his transition to leader, with a troubled past that feels quite a bit rushed but kinda works, Kylo gets his redemption - but he is still a doormat most of the time, Rey gets genuine moments of conflict and defeat - together with spectacular wins that at least are more contextualized than the previous movies, including VII who was JJ's doing.
The problem with most dramatic spots is that the movie dares very little, rapidly undoing any drama it sets up in "Gotcha, nothing happened" moments that are a bit cringy - any death in the movie is drained of emotional impact and carries no momentum. The old characters are mostly respected. A lot of old fans may have major issues with the finale and the overall outcome of the saga with the canonical arch of the Skywalker family - ironically, this is the rise of Palpatine more than Skywalker's. I did not leave with this impression, partially because it was kinda hard to take anything about the tradition seriously after the events of the previous two movies, with the weird unpopular choices made in VIII but also with the stunning choice to make VII a timid rehash of IV with paper-thin characters and a deep unbalance in the plot. In this sense this movie is the closest of the three to try and get the appropriate tone and make characters grow through actions, failures, conflict.
In the end, I think this movie does give a fitting ending to a trilogy that started probably with the wrong premise, giving us a menace that was too closely associated with the previous, a world inexplicably set back to step 1 undoing anything accomplished in the first trilogy. Perhaps Rian Johnson did not have a terribly bad idea doing away with all of that in episode VIII. It does appear stunning that one of the most beloved sagas in science fiction history has been produced without a clear, coherent planned outcome in mind, and this Episode IX is only the natural conclusion of it. The direction is more than competent, most of the problems lie in the plot and the need to cram too much in it.
★★☆☆☆
Sammo ★
Answer: It's a group of Sith cultists known as the Sith Eternal. They have been based on Exegol for many years.