What do devil worshipers believe about the afterlife?

That would depend, as there are two specific types of Satanist. There are the true ancient satanists, that are nearly as old as Christianity (Luciferan Brotherhood), but they are very secretive and it is next to impossible to find anything true about them. Then there are the LeVeyan Satanists who belong to the Church of Satan. They where founded by Anton LeVey in the 60s or 70s and are more of a moral code than a religion. They tend to believe that Satan doesn't care one way or another about you, and when you die you will go to Hell, though they never tell you if you will be greeted nicely or not. However I would suppose you would be on his side at the end of the world and if he won you would get Heaven and if not, basically exactly what you already had.