The Star Wars: The Last Jedi, with Kylo Ren murdering him and taking his place as ruler of the First Order.
Sith redoubt of Exegol. Since then, tie-ins have revealed Snoke was something called a "strand-cast," designed to be strong in the Force; he apparently had a degree of independence and free will, but Palpatine had total control over the Supreme Leader whenever he wished. Palpatine used Snoke because his own clone body was too weak and vulnerable, and thus he could not safely leave Exegol; instead he had to operate through a proxy.
Snoke's full origin story is finally revealed in Lucasfilm's 2021 book Secrets of the Sith, which purports to be an in-universe collection of Palpatine's own notes. It reveals Snoke was originally created as a potential host for Palpatine's own disembodied spirit, but his strand-cast body was unable to contain the Emperor's dark side essence; it's likely Palpatine's attempt to possess Snoke's body was the reason for the latter's disfigurements. This idea actually fits with the old Expanded Universe, which revealed the dark side has damaging effects on cloning. As explained in the Jedi Vs. Sith handbook, "because the clones are one step removed from the natural life process itself, they are much more vulnerable to the effects of the dark side, and age at an extremely accelerated rate."
This also explains why Palpatine had so many other Snoke clones in vats on Exegol; they were leftovers from experiments attempting to create a suitable host for his own spirit. It seems only two of Palpatine's experiments progressed to the point where they were released from those vats; a clone of Palpatine himself who lacked the Force, who left Exegol and became Rey's father, and Snoke. Palpatine thus pivoted, deciding Snoke could rule the First Order in his stead, while he waited for a suitable host. Secrets of the Sith confirms he was grooming Ben Solo to become that host, until he hit upon the idea of possessing his "granddaughter" Rey instead.
Star Wars: The Rise of Skywalker left a lot of questions unanswered, and it's fallen to a range of books and comics to clear up many of these mysteries. Secrets of the Sith finally clears up Snoke's backstory, and next year Lucasfilm will release a tie-in novel called Shadow of the Sith, by Adam Christopher, that promises to deal with the last remaining Star Wars mysteries and plot holes. This looks to be set shortly before Ben Solo fell to the dark side, which means it may well flesh out the final details - how Snoke met Ben Solo in the first place.
