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"I began a training temple. By the time I realized I was no match for the darkness rising in him, it was too late."

"I saw darkness. I sensed it building in him. I seen it moments during his training, but then I looked inside and it was beyond what I ever imagined. Snoke had already turned his heart. He would bring destruction, and pain, and death, and the end of everything I love because of what he will become."

These are the two descriptions Luke gives of Kylo's fall. Something of note is that Snoke was obviously not present at the Jedi Academy. Further, it seems unlikely that Ben was sneaking off in a starship during the middle of nights, going to Snoke, and the two secretly having dark-side talks without Luke really having an idea of what was going on.

This is just my theory, but I find it likely that Snoke - for most if not all of Kylo's seduction - was talking to Ben telepathically, perhaps appearing to him in dreams, visions, and the Force melding / projection we saw in The Last Jedi. If true, then I think in IX we could see flashbacks of such events taking place and elaborating on Kylo's fall further.

Thoughts?

That's the more sound reasoning and it seems the most likely.

If true, it also shows how helpless Luke was in the situation. There's nothing you can do if the enemy is hidden in the Unknown Regions.

I'd think Snoke might have been an acquaintance of Luke and the OT cast sometime after the OT, masquerading as a friend. Id also imagine that Ben while being a good hearted young man was burdened by hia family's legacy, and with Leia and Han gone doing politics and smuggling, Ben had no one to confide in. Id imagine Luke would have been more focused on grooming Ben as the ideal Jedi, rather than caring how he felt, secluding Ben more. That's where Snoke comes in and begins his seduction of Ben.

Well, that was my assumption, but the question is more like how did they meet and how did Snoke know to get into his mind to begin with?

Although it's probably the same way he knew how to bridge Rey's mind with Kylo's despite having never met Rey.

According to novelization, he was never told that Vader was his granddad, but Disney does their own thing.

Snoke may have pulled an Exar Kun and spoke into Kylo's mind