This is from an upcoming TROS tie-in book that I had an opportunity to work on in some capacity that I cannot disclose.
I
"Always two there are, master and the apprentice".
Sith aren't really just cool looking dudes with red lightsabers. Sith is technically a singular being, Sith Ari, a spectre that was born from a sort of a fission in the force that happened thousands of years ago.
"Master" is possessed by Sith Ari, while the "apprentice" is trained to kill their master so Sith Ari can take over their body.
Basically, all Sith characters are a single character that possesses different bodies.
II
"Strike me down, and and your journey to the dark side will be complete."
That was basically Sith Ari preparing himself to jump bodies and possess Luke, leaving the old Palpatine's body behind.
Luke refused, but the disembodied spectre, a part of Sidious' soul, so to speak, was haunting him for years nevertheless, pushing him to cut himself off the force.
III
Snoke was born only of a part of Sith Ari that latched itself to Vader. Obviously, he wanted to be complete again, that's why he was so obsessed with finding Luke.
Luke ascended in a "Prantu" ceremony to take the part of Sith Ari that possessed him away from the mortal plane.
IV
Ironically, when Kylo killed Snoke, according to Snoke's plan, one part of Sith Ari ghost now lives in Kylo, and now it seeks to find its other half beyond the mortal plane, where Luke took it.
I don't know what happens in TROS though. This extensive write-up will hopefully tell you what to expect, though.
Thank you for reading, cheers!