Zardnaar
Legend
Or they fully embrace the old Dark Empire comic and have Rey pretending to go evil in order to ingratiate herself with the reborn Emperor.
Luke did go dark side there, Leia dragged him back to the light.
There's various ways for them to do it.
1. Force vision.
2. Possession
3. Clone
4. She pretends
5. She actually goes dark (unlikely)
In the new canon Palpatine is basically Vader's father via the force.
Rey could be Anakin 2.0,in Dark Empire Palpatine tried to possess Leia's unborn baby and Sith spirits exist in the new canon and are very similar.