As far as the Knights of Ren are concerned, there was a rumor that they were in “The Beyond” (Unknown Regions?) this whole time and there they encountered a new threat.
Snoke’s obsession with Kylo Ren’s Skywalker bloodline and desire to make a new Vader certainly had parallels to Palpatine’s desires to turn Luke.
There is also a theory that Snoke was Palpatine, i.e. a host body ravaged by the power of the Sith Lord.
If true, Palpatine’s spirit could have been out there figuring out how to become even more powerful and he finally revealed himself to the Knights of Ren.