Continuity mistake: After the speeder chase, everyone lands in quicksand. In one shot, 3PO is buried headfirst. In the next, he has flipped and his upper body is visible.
Continuity mistake: When Rey enters the Emperor's vault on the Death Star she is wearing her satchel, slung across her body. When she falls out backwards after seeing Dark Rey she is still wearing it, when she stands to confront Ren, it's gone.
Continuity mistake: When Rey is shown for the first time while undergoing training in the forest she falls to the ground face-down. When she gets up her clothes are soiled with dirt as expected, but in all shots after that, in the same forest sequence, her clothes are perfectly clean again.
Continuity mistake: When Rey receives the talisman from the young alien who asks her who she is, the chain sits on the collar of her vest differently between shots.
Continuity mistake: Despite Rey having several struggles with the storm troopers and going through quicksand, when she gets to the cave her outfit is very clean.
Continuity mistake: When the First Order council is gathered, in the wide shot where Kylo Ren says, "We have a spy in our ranks...", he walks behind the council members, and General Pryde is nowhere to be seen. In the next shot, he appears out of nowhere, sitting opposite General Hux and glaring at the latter as Ren walks by him. In fact, in the first shot, a completely different officer is sitting in Pryde's chair, between a man with grey hair and General Quinn, the man that Ren slams into the ceiling. However, when Quinn says, "These allies on Exegol...", Pryde can be seen sitting in the chair instead. The previous occupant is much younger than Pryde and his face has a different shape. Plus after Reb says, "I sense unease about my appearance, General Hux", in the next shot, a completely different woman is sitting next to the man. The first one is an older woman with grey hair, while the second woman is middle-aged, with brown hair. As General Quinn says, "What is it, a gift?", the older woman is back. In addition, when the younger woman appears, the person sitting on her right can be partially seen, and it is a different person than earlier (a Caucasian person, as opposed to the black man that appeared next to the older woman earlier in the scene). (00:20:15)
Answer: It's a group of Sith cultists known as the Sith Eternal. They have been based on Exegol for many years.