The Un-Official Darth Vader Fan Club, also known as the Knights of Ren, was a Cult of Sith wannabes created by Kylo Ren to worship and finish what his granddaddy started. They are part of the larger Galactic Empire Fan Club and enjoy blowing up planets with them.
The club appeared to be unaware that Ren's granddaddy actually died destroying the Sith. This was strange considering Ren had met Luke Skywalker, who saw it happen. Possibly Luke forgot to mention it?