@Lilliana_Umbrellascope, thought this was an interesting bit of info that popped up on the Fandom.
The Sith Eternal was a secret Sith cult that followed the Sith religion, preserving it after the destruction of the final two Sith Lords, Galactic Emperor Darth Sidious and Darth Vader. As Sith cultists, they revered the power of the dark side of the Force and venerated Sidious, who cheated death by preserving his spirit in a cloned body. The Sith Eternal strove to prevent their master from dying again, and resorted to the science of gene splicing in hopes of building a more suitable vessel for his dark essence. Their experiments led to the creation of various artificial genetic constructs, known as strandcasts, such as Supreme Leader Snoke