Emperor Palpatine was behind everything. Rey is his grandaughter, her parents abandoned her to protect her. Palpatine's grand plan was to have her willingly kill him, whereupon his spirit would pass into her, together with the thousands of Sith Lords before him, and she would reign as Empress. He sucks the life force from her and Ben, who has returned to the light, restoring his body, and fires lightning into the sky, disabling the Rebel armada which has come to defeat the Final Order fleet. Rey drags herself to her feet and using crossed lightsabers deflects the Emperor's lightning back into him, destroying his body utterly, but mortally wounding herself in the process. Ben uses his last bit of strength to heal her wounds, bringing her back to life. He smiles and fades away. Finn destroys the lead ship of the First Order, and the Rebel armada makes short work of the rest. Rey returns to Tatooine, where she buries Luke and Leia's lightsabers together. Asked her name by a local woman, she sees the force ghosts of Luke and Leia smiling at her, and she declares herself as Rey Skywalker.
Jon Sandys ★
Answer: It's a group of Sith cultists known as the Sith Eternal. They have been based on Exegol for many years.