The Aftermath series deals with the years after the fall of the Empire, and within those books is a strange organization of Darth Vader worshippers known as the “Acolytes of the Beyond,” which are outlined in detail in this video from Star Wars Explained.
Part of the joy of the Star Wars universe is that it’s not just made up of a series of films.
For every cool new character The Force Awakens introduced and every familiar face it brought back, ten new questions popped up in their wake.
Long story short: The Acolytes believe Vader isn’t actually dead, wear masks, and are obsessed with collecting artifacts from the war against the Empire.